To go back and "undo" an exercise in your Personal Training section, simply go back to the exercise or section you wish to undo and follow the directions to double-tap the screen. Once you double-tap the screen, you will be able to double-tap each exercise you want to un-do. Your points will adjust down accordingly!
To undo the logging of an activity, simply go to your Profile and tap the 3 dots along the bottom. From there, you will see an option to undo the logging. Tap this and you will be all set! Again, keep in mind that your points will reduce once you take this step.