Steps for downgrade:
- Download and extract T-MST12DEUC.zip or T-FXPDEUC.zip (according your TV model) to root of a FAT formatted USB drive:
Install it following usual Samsung official procedure for USB upgrade:
How do I upgrade my Samsung TV's firmware via a USB memory drive?
- Then repeat upgrade procedure with any other version of your choice. Firmware list you can find here: viewtopic.php?f=70&t=9497#p77431 (check spoiler, donor's access only)