1. Select 
Groups
2. Select the group you want to leave
3. Select Settings
4. Select X Leave this Group
1. Tap 
Communities
2. Select the group you want to leave
3. Tap 
4. Scroll to the bottom and select Leave this Group
1. Tap Communities
2. Select the group you want to leave
3. Tap
4. Scroll to the bottom and select Leave this Group