After training Keen for a particular hand, you may remove the training and change to the other hand.  Open your Keen app and tap on the appropriate Keen in the app to manage it.  Tap Manage again to open up the Keen.  On the bottom of the screen you'll see a Delete button.  It will ask you to first delete any trained areas, and then you will be able to delete the Keen and re-start training on the other hand.  

To delete a trained area, follow the instructions on the Manage Band screen and swipe on the trained area.  You may also tap on the trained area, scroll down, and tap the Delete button.