If you want to select or deselect a room as a favourite, you need to enter the room you want to mark as a favourite. In the upper right corner next to the graph, you'll find a star symbol. A grey star means that the room is not a favourite, but when pressing the star symbol it turns yellow and the room is now a favourite.
You can go back to the star tab on the overview in order to check if the room has been added as a favourite.
Once you have starred a room, you will now receive all notifications and team messages for that room.