You have two ways to unassign a SentriLock lockbox from a rental.
Reassign to a different rental
To move a SentriLock lockbox from one rental to another you can just click into the rental settings of the new rental, turn on SentriLock self-access viewings, and pick the serial number from the dropp-down list for SentriLock serial numbers. The serial number will appear as long as the rental to which it is currently assigned to is inactive.
If you don't want to put the SentriLock lockbox on another rental yet, go to the SentriLock lockbox page and click Settings under the appropriate serial number. Then click the Unassign button at the bottom right of the SentriLock lockbox settings page.