Some of my daily findings while assisting IT Pros. Hope its helpful, feel free to ask for help, I will be delighted to assist where I can.
It was not very easy to find the firmware so I am putting the URL online for anyone who needs it.
ftp://ftp.aztech.com/Support/MALAYSIA/ADSL/DSL600E
both upgrade and downgrade is easy, just follow the instructions in the package.