In the case that you would like to leave your group, you can do so by visiting Teams and Group on the left-hand side navigation. 

In the list of Groups, you can click on your Group.

Here, please select "View Group Profile" by hovering over the group profile image: 

Now, you are able to select leave the group on the drop-down button by your profile picture: 

You will then receive a confirmation message, just to make sure you want to leave your group.