Challenge Re-roll is available for players that have an active Pro Pass subscription. With every Challenge refresh (daily), you will get 1 re-roll. This means you can switch one of your Daily Challenges if you don't like it. You can only do this once a day, so choose wisely!

To re-roll a Challenge, just click on the "refresh" button on the corner of the challenge box (image below).
Please note that if you re-roll a Challenge, all the progress you've achieved in that specific Challenge will be lost.