You can log into the Master Lock Vault web portal https://enterprise.masterlockvault.com/
Click on the lock box to
open the detail page, where you'll see a Delete option.
Once the box is deleted from the account, it can be registered to the other person using the card that came with the lock box by using the Master Lock Vault app and scanning the QR Code with their phone. It can also be loaded using the web portal, but the app is quicker.