Can I quit a challenge already in progress?
Yes. You can leave a challenge penalty-free on the first day. After that, you can quit by selecting "Give up" and paying a flat $20 fee (goes to Spar) in addition to any incurred penalties (goes into the pot).
How can I leave a challenge?
To leave a challenge you have joined, penalty free, within in the first day of the challenge, swipe up and tap "Leave Challenge"
How can I Give up?
Swipe up on the challenge page and tap, "Give up"