You need to bring the rental bike back to its original pick up location before your rental period expires. Why? Because somebody else may have booked that bike after you and they will look for it in that location -marked with a red pin in the app's map.
Once you're in the right location, park the bike within 10 meters of the place you took it from and lock it. Double-check that the lock stays closed. Then open the app and tap END RENTAL. Thanks!