You can upgrade the museum with the Roost Café. Just ask Blathers how to find Brewster, and go on a boat tour to seek him out.
How to Unlock The Roost Café
Update Your Game
Before you can unlock The Roost, you need to update your game to 2.0.
Before you can open The Roost, you need to have hosted at least one K.K. Slider concert on your island, you need to have unlocked art donations at the museum, and you need to have donated at least one thing to the museum.
Talk to Blathers
After you update the game, go into the museum and talk to Blathers, and agree to help him find Brewster. You will get Brewster's Photo, and Blathers will tell you to go on a boat tour.
Talk to Kapp'n to Go on a Boat Tour
Go to the pier on your island to find Kapp'n. Talk to him and pay 1000 Nook Miles to go on a boat tour. You can only go on one boat tour per day.
Talk to Brewster
Look for Brewster walking around on the island that Kapp'n took you to. Talk to Brewster and he will give you a gyroid fragment, and will say that he will see you again soon.
Talk to Blathers Again
After you return to your island, talk to Blathers again and he will say that he can build The Roost. The museum will be closed the next day, and the café will open the following day.
After the Roost opens, you can talk to Tom Nook in the Resident Services building to hold an opening ceremony for the café.
Talk to Blathers
The first time you talk to Blathers after the café is open, he will thank you and tell you a little bit about the café.
If you have completed any sections of the museum, talk to Blathers again after the café opens, and he will give you posters for each section of the museum that you have completed. You can talk to him again to buy more copies of these posters.
Visiting The Roost
To visit the Roost, go into the Museum, then go up the stairs behind Blathers and go through the new door on the left.
Sit on a stool in front of Brewster in the café, and you can spend 200 bells to get a cup of coffee. Keep getting coffee each day and Brewster will start offering new services.
Inviting Café Visitors with amiibo
The Roost Café has a phone that you can use to use an amiibo or an amiibo card. The character on the card will visit the café. Only one character at a time can visit, so if you use another amiibo, the character currently visiting will leave.
Coffee To Go
After you have gotten coffee on three separate days, you will be able to buy coffee to go, starting the following day. Just talk to Brewster at the counter on the right to buy coffee to go.
After you get coffee a certain number of days, Brewster will give you a special gift as you exit the café.