well i did exactly what you severusx saying, hav the PUP file like you describe and after i try to "system update" with the 3.55 OFW in a flash usb in the first usb port from the right it says: checking,please wait and then it stops and gives me a message:
A serious error occured. Please contact technical support for help (8002F281).
and i am sure i have the right 3.55 OFW beccause i used it once and update it with the same and it went well. but after i mistakely tried to downgrade it with a usb dongle (which i`ve done it once without any problem) with a .mHex from 3.50 to 3.41 because i could not find from 3.55 to 3.41 and got stuck in the middle of nothing. i tried again to update it and managed to get out of the red screen like i wrote it by having the Lv2diag.self and the 3.55 ofw in a root of usb and then managed to start updating to 3.55 halfway and after that i am stuck in a recovery mode.
Sorry for my bad english, lol.