Thank you for posting your question. Please visit adtran.com for firmware/software downloads. Here's a direct link to the software download landing page: Software Downloads
There's no cost for firmware updates. It is recommended using the latest Extended Maintenance Release unless you need a particular feature in a later release. At the time of this post the current extended maintenance release is R11.4.2 for 2nd and 3rd generation 12XX switches.
I marked Chris' answer as assumed correctly answered.
If you have more questions, feel free to let us know.