Once you select Chat from the top menu bar, you will be brought to the Chat page.
All of your chatrooms will be displayed. You can select the chatroom which you want to leave, and click on Open Chat.
You will be brought into the chatroom. At the top of the chatroom, locate and click on the stacked 3-dot icon. This opens up a drop-down menu. Select Leave.
Once you have completed that step you will no longer be a part of the group you just left, but are always welcome back!