To leave a challenge, open up the project it’s currently attached to. You can do this by clicking “My Novels” in the main navigation bar, going to your View All Novels page, and then clicking the novel’s title. Then, click the wrench next to the challenge’s name in the “My Word-Count Challenge” box. Click the “Leave this challenge” link in the pop-up box, and enter your password to confirm.
To change a project you want to use for a challenge, first follow the steps above, then accept the challenge with a new project.