To end your rental session, you must bring the bike back to the original chain you unlocked it from.
1) Return to original rack
2) Locate the chain you unlocked the bike from, and wrap it around bike
3) Push the button, wait until it turns blue
4) Insert the chain into the top hole and hold it in
5) Press "Lock" in the App
You should see the light turn Green, which means you are finished.
To end your rental session and pay, tap the Red button and you're good to go.