From the top right options of the individual shift page, select either "Cancel Job" or "Report Issue". Cancelling irreversibly removes a shift, while reporting an issue notifies the Outside team. Hirers may also cancel shifts as well.
Do note that agent-side cancellations may negatively impact your community rating. Hirers assess your rating when you send in offers, and a low rating may also result in an account ban or suspension.
Find out more about Outside's community rating
here.