How do I force-quit an individual app on Apple Watch?
For watchOS 3 and newer:


Press and hold the Side Button 
Press and hold the Digital Crown


For watchOS 2:

When the app that you would like to terminate is open:


Press and hold the Side Button and wait until the shut down screen appear. Then let go of the side button.
Press and hold the Side button again until the home screen appears.


Your formerly active app has now been terminated.

PS: The Side Button is the button below the Digital Crown.