Go to School > Timetable > Timetable Administration > Timetable Slots.
On the Timetable slots page, you are able to take several actions, use the filter panel at the top of the page and filter your timetable slots by subject, course / class or year group. This will help you easily identify lessons that have the same teacher, or are in the same room.
Tick the lessons that need to have the room changed and then click on the green pen. From the list of options click on "Change Rooms".
The "Add/Amend Rooms" slide over will then load with a drop down that populates all rooms setup on the system. Click on the room you want to add/amend and then click on "Change Rooms".