From the Inventory menu, click to view the Availability grid.
The legend above the dates shows the colour coding for the different availability statuses.
To close out the Room Type, click on the row for that room on the date you want to start.
You can set the date range and then change from Available to Unavailable. (Note that Closed has a special function relating to Channel Manager in Caterbook and should NOT be used for this purpose.)
Click Update, and your changes will be made.
Note that you could have selected All in the Room Type drop down if you wanted to change the Status of every room type.