I am not sure if you got the information yet but sharing it if still needed.
9.12.xx >> 10.0.0.0.1424 >> 10.2.1.0.147 (Recommended version) >> 10.4.0.0.70 (Latest version)
For downgrade, please keep the backup file downloaded in each upgrade and you can use same if you want to downgrade back later.
Please note that downgrading beyond one step back (very last version which was running before upgrading to current firmware version) will factory reset the controller and you have to reconfigure it from the scratch.
Syamantak Omer | Community moderator | Staff TSE | CWNA | CCNA | RASZA | RICXI | Follow me at https://linkedin.com/in/syamantak-omer-5949b6124