Go to the group page and click the ... menu → Leave Group. Your membership ends immediately.
What Happens to Your Content
Posts you created remain visible in the group unless you delete them individually before leaving
Comments you left on other posts also remain
Reactions are removed automatically
If you want a clean exit, delete your posts first from your profile's Posts tab filtered by group.
Owner Transfer
If you are the sole Owner of the group, you must transfer ownership before leaving. Go to Group Settings → Members, find a trusted member, and click Make Owner. If no one is available to take ownership, you can Delete Group instead, which permanently removes the group and all its content.
Leaving a group does not block other members. If you want to stop seeing someone, use the block feature from their profile.