After you battle a Security Robot in sector 3, Adam tells you to go to the sector 6 data room.
Make Your Way to the Data Room
Get your new briefing in the Navigation Room. The Varia Suit data is ready to be downloaded, but X from the sub-zero ARC sector have entered NOC. They have been changed by the cold, and if you touch one, you'll be frozen from within and damaged. They can be stunned by your beam.
There are many small Eeleyes in the next room, easily defeated. Kill them, then exit right to the next area. Drop to the bottom level here, and jump and bomb the left wall between the rocks jutting out to open a passage. Go through and you reach a room with a Missile Tank. Get it and go back out to the last room.
Use three Super Missiles to destroy the barrier, jump on the ledge, then jump and bomb the right wall between the jutting rocks to open another passage. Go through to a new room. From the ledge you start on, jump as a ball and plant a bomb near the top of the right wall. Jump into the top passage (you may have revealed two more if you kept bombing; neither the middle nor the bottom allow you to reach the Energy Tank), roll to the end, and bomb to reach the Energy Tank. Return to the last room where you got rid of the purple barrier, head upwards and exit through the door above.
There is a blue X here, shoot it to stun it, and quickly make your way to the right and exit. Shoot this blue X, and quickly head down. Ignore the first two doors on the way, there are tons of blue Xs beyond them. Exit through the door at the very bottom of this area (the blue X you stunned may float to you as you get to the bottom, be careful to avoid it).
Up ahead, don't shoot the bottom block, as it hides a blue X. The top two are safe to shoot, so get rid of them and continue on. Get on the ladder to the right, climb a little ways down so you're across from a thin wall, and shoot to the left to break open the wall. Go through, jump over the gap, shoot the blue X, and fall below to go through the bottom left door.
Jump up to stun the blue X here, then make a running start towards the left so you Speed Boost through the wall ahead and end up on a ladder. Aim right and shoot the blue X as they head towards you. Then drop below and quickly go through the door to the right.
Get rid of all the Eeleyes, exit right to the Save Room and save your game. Then return to the room where you Speed Boosted through the wall, continue left from the door and quickly exit through the door to the left, avoiding the blue X.
Grab the Energy Tank, then bomb to the left of where it was to break open a hole. Drop below, roll to the right to fall below and find a new door; go through. Bomb the right wall to fall into the narrow space below, then roll to the left. The SA-X enters the room. Stay where you are, and wait to hear it set off a Power Bomb (if you follow it to the right, you'll get hurt by the bomb). Wait to hear the SA-X shoot its Ice Beam and exit the room. Then jump up through the floor it destroyed with the Power Bomb, and head right. If you don't jump, you'll fall through some Pit Blocks down below (which is fine, unless the SA-X is still there). Exit to the next room.
Get rid of the Eeleyes (the grub-like things will fall off the wall when you shoot them, so be careful; you can leave them alone), and shoot out the left wall. As a ball, jump into the passage and roll through. Exit through the left door.
Exit left to a Save Room, then go back to the last room. Shoot the block in the middle of the floor to fall below. There are blue Xs hidden in many of the blocks here, but you'll be getting a red X soon. Still, be careful. Use a bomb at the lower-left corner of the room, leaving the block above it alone (it hides a blue X). Roll as a ball to the left to enter a new room. Destroy all the grub-like monsters and pull up into the ledge. Bomb the far left to access a Missile Tank. Return to the room with the hidden blue Xs.
Shoot the square block above, then stand on the rectangular block in the center of the room. Shoot the wall to the right (hopefully not revealing the blue X at the far right), and go through the door.
Kill the Eeleyes and jump over the wall to the Living Door. Destroy it and go through to the Data Room, where a Core-X appears to somehow be downloading the Varia Suit data. Don't touch it; it can hurt you. When it leaves, exit right and it will attack you.
The only way to hurt the Barrier Core-X is to hit it with charged shots; Missiles do no damage. Avoid it if you can and keep hitting it until it turns into a regular Core-X. Kill it as usual, then grab the Core-X to recover the Varia Suit.
Sector 6 (NOC)
Now you can survive extreme temperatures. Touching a blue X will now recover 30 units of energy (though they may flee from you now that they can't hurt you anymore). Exit through the door to the right. Keep going right to a Save Room, then return to the last room. Climb up the ladder, collecting blue Xs, and exit through the door on the left.
Don't walk directly to the Missile Tank or you'll fall through some Pit Blocks. Jump to it to get it. Hold left after you fall to grab onto a ledge, and pull up into it. Roll to the left, then shoot the middle ceiling to reveal another Missile Tank. Roll back into the shaft and fall below, then exit right.
Climb back up (the blue Xs will probably have wised up by now), and exit right this time. Get rid of the blue floating enemies and grub-like enemies, and climb up to the door above. Go through.
Get rid of the Owtch, roll through the tunnel and you'll fall through a Pit Block, then roll through the next tunnel. Make your way to the top left door of this room and go through. There are a couple of doors here you should have avoided because of the large numbers of blue Xs inside. Now that it's safe, ignore the first door on the left (dead end), and go through the second door leading right. Go through, collecting blue Xs and killing any grubs in the way, and exit right. Don't immediately go for the Missile Tank; when you approach it, it sprouts wings and reveals itself to be an enemy. Destroy it with a Missile, then bomb the right wall to open a passage to the real Missile Tank.
Make your way to the Navigation Room at the entrance to Sector 6, and uplink for a new briefing. With the Varia Suit, the SA-X can no longer freeze you. You still can't hurt the SA-X with your current abilites, but the Plasma Beam may be able to hurt it. HQ will work on that addition, and it's possible you can regain that ability from a Core-X. Your cellular makeup post-Metroid vaccine would cause you to reject an Ice Beam ability, but HQ has developed Ice Missiles to deal with that. The data can be downloaded in Sector 5 (ARC).
Head to the elevator down to ARC.