Go to Resource Planner > Find the class
Click on the group activity > Click on the "pen and paper" icon to view the booking
Go to the "Base information" tab
Click on the room resource
Click on the arrow down next to the room to choose a new one
Click on "OK"
If the room you wish to switch to is not in the list, it is because you didn't add this room to the group activity product under "Resource requirements" in Configuration. See the article below on how to add this to the product.