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