Within a campaign navigate to the Manage Units tab
Use Filters or Face ID search to narrow down units to desired list of units
Use the Toggle box on the right hand side of this view to select desired units to mark as booked. The top Toggle allows the ability to select all units.
Once selected, then click on the 3 vertical dots at the top to expose the Bulk Action dropdown.
Lastly, select option for Mark Selected Units as Booked