If it's asking for a dealer number, you're doing something wrong, probably trying to register for an account. You don't need to do that.
The updater is a windows application, so you must be able to run Windows. I have it in a virtual machine using Parallels on my Mac, but there are other ways to do it.
From inside Windows, launch a browser, go to
www.xtooltech.com, set the language to English, and scroll down a little bit until you see the image for Update Tool. Click on that. It will download the setup program (setup_en.exe) to your computer.
Once the file is downloaded, launch it. It will run the installer. Once the installer completes, click the "Run UpgradeTool" button.
Remove the microSD card from the VAG401, insert it into the microSD reader that came with your VAG401, and plug the microSD reader into your computer. Make sure your virtual machine connects to the reader (it should show up as a removable drive).
When the upgrade tool launches, it will prompt you for a username/serial number and a password. The username is the serial number from the back of your unit (I entered vag in lowercase). The password is found on the "About" screen on your unit and is 8 digits long.
Once you log in, the program will show which updates are available. For me, right now, it's showing version 8.41 for "VAG401", dated 8/10/15, and version 7.0 for "DISPLAY", dated 9/20/12. Check them both, make sure the drive letter for your card reader is shown next to the SD Card drop-down, and click the Upgrade button at the bottom of the window. A new window will open, and the upgrade files will download (very slowly, if my experience is any indication).
When the download finishes, a window will pop up saying, "Upgrade files which have been downloaded continue?" Click the Yes button. It will copy the files, then say "upgrade successfully." Click OK. Close the updater, eject the SD card, and replace it in your VAG401. You're now upgraded.
I can't help with the fuel filter replacement as I don't (yet, anyway) have an A6.