If you want to debrand it, you should use GENERIC firmware:
K850_R1FA035_MAIN_GENERIC_SO_RED52
K850_R1FA035_FS_WESTERN-EUROPE_RED52 and as a custom pack
K850i_1201-0077_R11A_GENERIC_WESTERN-EUROPE
Read this tutorial:
http://www.topsony.com/forum/a2uploa...tml#post111079.
As you see, you can also flash custom pack if it's a winrar.zip archive and you put it to "MISC files"