As a member of a community, you can leave the community if you no longer wants to be a member.
A community must have at least one owner. If you are the last owner, you will not be able to leave the community. See how to transfer ownership
Go to Members-tab in your community.
Find your own membership (marked with a You-tag) and click the red Leave-button.
Confirm you want to leave by clicking the Leave-button in the dialog. IMPORTANT: Rejoining the community requires that an owner/manager sent you an new invitation to join.