You can only change the bookable status of a date if it’s within the current booking window. To update dates beyond that window, you’ll need to expand your booking window (for more on that, see: How far out can Hipcampers book?)
In your Host Dashboard under the "Calendar" tab you can change the bookability of a single day by first selecting the day. You will then see a pop up displaying your listing(s) and corresponding sites where you can mark as "Bookable" or "Closed". To close an individual site, select the corresponding box to switch it to "Closed".
If you want to adjust the bookability for multiple listings and/or sites at once, click "Change all":
To close multiples days at once, click on the starting date and without releasing drag the mouse to the last day of your range:
If you choose to close all of your sites you will see the days closed off with gray diagonal lines and "All sites closed" in the top right corner of the days: