You can return the bike to any station with an open dock – you don't have to bring it back to the station where you picked it up.  Use the slider on the top left of the map screen to toggle between available docks. 

Tap on a station to see the exact number of bikes and docks available. Once you’ve found an open dock, line up the triangle located at the front of the bike with the dock, then insert your bike gently but firmly, holding the bike in place for at least 5 seconds. Please don’t slam it! When the light on the dock turns green, your bike has been properly locked. Just keep in mind, it can sometimes take a few minutes for a dock to register that your bike was returned.