Settings > Rooms & Extras > Then click on 'pen' icon to edit specific Room Type and down you'll see room numbers. Just change number you want and Save on bottom.
Note that if you added more room number than you originally had your availability for that room type will be increased accordingly. Same goes if you remove more room numbers than you originally had - availability decreases.
If you are interested in How to close availability / closure for specific period of the year - click here.
And for How does the room availability change - check here.