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