I think Android will see it as an update for the app, which means it'll keep any local storage for that app, so you'll keep your funds. However, just to be sure make sure to backup your seed (go to the menu, backup, and follow the process). You should always have a copy of your seed somewhere so you can recover your wallet in case you lose your phone or it gets stolen. If you do need to recover your wallet, you can choose "recover from seed" when creating a new wallet and enter your seed.
Make sure to get the .apk from Mycelium themselves and not from a 3rd party
.