Open the Group that you wish to leave
Click the 3 dots icon in the top right corner of the screen to display options
Select 'Leave Group'
[Note: If you are the last Group Member you will not be able to leave the group. The group will need to be Archived]
Click 'Yes' to leave the group
[NOTE: You cannot leave the group if you are the only ADMIN of the group. You must assign someone else as the group ADMIN before leaving] Please see How to Archive A Group if the group is no longer needed