If you want to leave a group that you are part of, follow the below steps:
  1. Go to the group
  2. Tap on the three dots from the right upper corner
  3. Select Leave Group
Leaving a group will cut your access to the content of that group and of your ability to make posts or comments, if the group is Public, you can alway rejoin the group, but if the group is Private, you will need to be invited to be able to join again.