Simply download the DMS Tools, switch to DMS Tools Mode (click on the DMS Tools Mode button)
Select your DMS in the dropdown
Select the firmware/version combination you know to be on the DMS
Uncheck the option 'Reflash Box'
If your box is not hacked, ensure the option 'Install non-hacked firmware' is checked
Ensure the option 'Install AAC' is checked
Click on 'Upgrade Firmware'
Check out the DMS to make sure it still works in the car.
Make sure you are running the latest version of PhatNoise Media/Music Manager (if necessary download it from the PhatNoise.us) site which provides AAC file support and you should be good to go!