If you would like to exit a chat room, tap on the three vertical dots at the top right of the screen and click on 'close room' when it appears.  Or you can 'back out' of the room by clicking on the < icon at the top left to go to the home screen.  The room you are in will be highlighted in the color green at the top of the home screen.  Tap on the 'X' on the top right of the room panel to exit the room.