Download the latest firmware from the Dell Support Page or the FTP.

Rename the *.exe to a *.zip and unpack it to a local destination of your choice.

For the update only the *img file within the "Loader_FW"-folder is needed.

Stop all running jobs of your backup software and wait until the display shows „System Ready“.

Connect to the autoloader via the web interface and sign in.

Standard user: admin
Standard passwort: password

Firmware -> Browse -> Upload

Select the location of the extracted *.img file. Click Upload and the PV124T will do the rest of the update.
You can update the firmware for the drives too, but this can take a long time. A faster solution for the drives is to use the update function in ITDT.