Some providers offer the possibility to make changes directly in your property management system (PMS), which is linked to Booking.com. If your provider supports this option it will reopen availability automatically, and the unit will go right back on sale on Booking.com.
This option is called Reporting API. Check with your provider to see if it’s possible.
If this is isn’t supported by your provider, you can log into your Booking.com Extranet to make these reservation date changes. These changes will be reflected in your Booking.com availability and in your property management system (PMS).