Made a mistake when manually adding an activity? No problem! We've all done it!
You can delete manually-added activities by tapping the Menu icon, then Activity, then opening the activity in question.
One thing to note is that your Active Minutes and Good Points count will not decrease when you delete an activity. (This is to prevent everyone in your challenge from seeing "negative" progress.)
Instead, you'll have a gap in your account (behind the scenes). Your new activity will count toward that gap, and once you meet that surplus, your totals will start to go up again.