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The first thing to do is download the firmware from our website. Make sure you are logged in. If you are not, then you can register and make a login without having to leave the website --- its pretty fast. Once you are logged in, I would enter the part number for your ADTRAN product in the search bar (top right hand side of the webpage) and hit enter. The first hit should be the product page for that unit and you will want to click on that. Once the product page comes up:
Click on the white Support tab in the middle of the page
The page will reload and you may need to scroll down a little
Then click on the Total Access 9xx link under Software Downloads on the right hand side of the page
This link should be below the Support Community box in the middle of the page
Now you should be able to see all the firmware available. Download the one you want and use this guide below to upload the firmware from the Command Line or the Web GUI.
Either method is covered step by step. If you have any questions feel free to reply to this post.