If you want to leave a group, the process is pretty straightforward. Navigate to the group you want to leave. You will see a button right under the group's image that says "Leave Group". If you click that, you will get a notification asking if you are sure and accept.
When you leave a group the chat thread and all corresponding events will be deleted from your app. If you decide to re-join the group, you will start fresh with events and group chat.