Downgrading to 5.03 M33

[O-bake]

Hi,

i haven't touched my (FAT) PSP in a while, so I need a little help.
I have 5.03 M33-3 installed and i want to upgrade, but the official site says if I have the fatmsmod, i should reinstall 5.03 M33-4 and then update.

How exactly am i gonny do this?
And how can i find out, if I have this driver installed? I can't remember.

[PSP FR34K]

where did u get that firmware from