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. When you arrive at a dock, line up the triangle located at the front of the bike with the dock and insert the bike gently but firmly, don't slam it! Wait until the light on the dock turns green to make sure it's locked. If you aren’t sure, pull the bike by the handlebars to confirm it’s properly locked. Please always make sure the bike is docked at a Bluebikes station when your ride is over; never leave it unattended.