Open the Project page → tap "Drop project"
Select a reason from the dropdown
Confirm
Once confirmed, you're unassigned, the Project is released for reassignment.
Consequences
Too many last-minute unassignments make your rating drop
Repeated unassignments → reduced access to Projects, temporary restrictions, or suspension review
Only unassign if a serious and unforeseen event makes it impossible to complete the Project. Scheduling conflicts you could have anticipated before accepting are not a valid reason.