Tap the Info icon inside the Circle to open the Circle Info screen. Tap the Leave Circle button. Leaving a Circle will remove all your contributions (photos, videos, comments, and reactions).
PhotoCircle Essentials and Enterprise
PhotoCircle Essentials and Enterprise users can leave/inactivate their circles without removing their contributions to those circles. Users can rejoin/activate the circle later by tapping the + button on the main screen and then tapping Join Group Circle to load the available list of circles.