You must return to official Sony firmware before installing the new custom version. How To Update Your PSP TO 6.61 Official Firmware
: Download the 6.60 PRO-C2 (or PRO-C Fix3) files. psp 352 m33 upgrade to 660
The safest method involves first updating to and then re-installing custom firmware (CFW) like 6.60 PRO-C or ME-2.3 . Prerequisites You must return to official Sony firmware before