You need to go to UserCP - a link is provided at the top right hand side of any page. Look for 'Miscellaneous' and second option down is 'Group Memberships' - click this link.
You will be presented with a list of usegroups that you have subscribed to or are still available. Find the group that you wish to leave and use the leave group link.
Please note the staff will not tolerate people flipping between groups - you are permitted to leave only once and may be rejected if you try to re-join later on.