After defeating Gadrongo in the Bowhole and obtaining the Wyrmlight Bow, you can reach Upover.
Bridge the Gap
Zoom to Wormwood Creek, then go directly west to reach the area in the southwest of the Eastern Wormwood map. You will reach Wormwood Canyon. Gather some Brighten Rocks in the northwest area, then go to the end of the narrow strip of land pointing southwest.
Stand on the dragon symbol and examine it. Say yes to fire the Wyrmlight Bow. After a scene, walk over the bridge to reach Wormwood Canyon.
Go to Upover
West of the bridge, you can gather Wakerobins to the north.
To continue on, go south to reach the Wyrmtail Region. Continue south past the fork in the path (you can go to the end of that path if you want to gather tanglewebs) and make your way around the big spiral.
To the south, you will reach the Wyrmwing region. If you go to the far south of Wyrmwing, you can reach an area where you can gather Thunderballs. Otherwise, go west. At the southernmost point of the beach on the west side of Wyrmwing, you can find glass frit.
Exit Wyrmwing to the northwest and you will be on the west side of Wyrmtail. Go northwest, then go northeast when you can to enter Wyrmsmaw.
From where you enter Wyrmsmaw, go northwest. As you go along, you will pass some stairs on the right that lead up to a ledge where you can gather lava lumps.
Continue going northwest. Eventually the path curves toward the east. There will be a path leading north, but this won't take you to Upover, so skip it for now. Instead, go east until you can't go any farther, then go south. The entrance to Upover is in the center of the map.
There are recipes and blue chests and other things scattered around town. After visiting the buildings, go up the stairs near the inn. There is a guard at a cave and he won't let you pass. Go into the house here and talk to the mayor. Go outside, then after Stella says some stuff, talk to the guard at the cave and he will let you pass. Go inside to the Magmaroo.