How do I change the difficulty of a specific challenge?

Every Saturday lunchtime, you will be served your challenges for the coming week. You can change the difficulty of your 1 player at the bottom of Your Challenges page, before Sunday at midnight, but once a challenge has begun, you cannot change the difficulty.  If your 2 player challenges are too easy or hard, you can go into your Settings to edit your Activity preferences and difficulty for the following week.