If you want to leave a Challenge, navigate to the "Challenges" tab.
Tap on the Challenge you want to leave.
Now tap on the small arrow to the right of the Challenge name.
In the next screen you can leave the Challenge.
If you want to leave a Challenge, navigate to the "Challenges" tab.
Tap on the Challenge you want to leave.
Now tap on the small arrow to the right of the Challenge name.
In the next screen you can leave the Challenge.