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.