There are many reasons wht you would like to change a room in the same room category, for example when the room that was originally allocated is not yet cleaned and the guest has already arrived.
On the booking page of the concerning booking go the the Room/ Allocation section and click on "Reallocate" :
In the pop up screen that appears choose a clean room, or the room number that is desired:
The system will tell you whether it is successfully done:
Go out of the pop-up screen by clicking on the x in the left-hand top corner.
The room number appears changed: