This must be done in Bluetooth range of the Padlock with Bluetooth and Internet connection.

Tap on the menu icon on the top left hand corner of the app
Tap on 'Home'
Select the property where your lock is associated to.
Tap on the lock you wish to delete
Tap on 'Settings' at the bottom right hand corner of the screen
Scroll down and tap on 'Delete Lock'.
Ensure that your phone is near the lock (less than 1m)