When a software update for your Smart Lock is available you get notified in the Nuki smartphone app. You can also search for updates in the Smart Lock settings. Tap on the desired Smart Lock in the Nuki app and enter the lock settings. Then go to "Manage Smart Lock" and choose "Firmware update". Then follow the instructions in the Nuki app to update the firmware of your Smart Lock.
In some cases, some smartphones may fail to transfer the new firmware to the Smart Lock. If this occurs, simply click "Retry" in the popup. If the transfer still can not be completed, remove the batteries from the Smart Lock and reinsert them. After the Smart Lock has rebooted, start the firmware update again. Now the transfer should work.