From the class list view, you can click 'Edit class' on a tile and then select your room choice from the "select current room" drop down menu. Previous room selections for this class will appear underneath the drop down menu and can be switched to with a simple click.
You can also assign a class to a room from within the class. Start the class and click on the 'Rooms' tab at the top of the page. From here you can scroll through a list of the classes, simply click one of these to assign it to your class.
WARNING: If you select a room that you have not created, please be aware that any changes you make to that room will affect any staff members also using that room and can result in the loss of their layout. Please refrain from editing a room that is used by other staff if they are unaware you are going to do so.