If the battery isn’t swollen just glue it down with some gorilla glue I did this with my S0 and it’s fine, I wouldn’t trust it being waterproof anymore.

If it’s a swollen battery you need to be super careful because it’s a huge fire hazard. Don’t charge it.

r/spicypillows

This is a very easy repair, the battery is held in with just one screw, a new one costs like 15$. To close it back up you need an adhesive strip, you can buy those pre cut and ready to install for about 10$ shipped. Any phone repair shop can help you with that, or you can do it yourself. There are great video tutorials online explaining every step.

Screens usually detach like that when the battery has substantially swelled. I can happen for other reasons, but this is the most common. Assuming the battery is swollen, a) be very careful with it and you should properly dispose of it as soon as possible, b) consider buying a new watch.

It’s not the adhesive around the display that has loosened; it’s the swollen battery pushing it out.

Replace the battery, then glue the display back.