For Android, you need to download the newest APK and install it over the old version.
You don’t need to delete the old app first. If you delete it, Android may remove your save data too, so only use Update / install over the existing app.
There is also RenPy’s upload sync feature, so you can back up your saves before updating just to be safe. In the game, use the sync option from the save/load screen, upload your saves, then after updating you can download them back if needed.