You are using tools like modoru to downgrade your firmware to 3.60 or 3.65 (the "golden" firmwares for homebrew) and need the specific version file. How to Install Firmware via PC (Manual Method)
Running RetroArch or backing up physical game cartridges. psp2updatpup
Ensure you get the correct version of psp2updat.pup from a trusted source (official Sony servers are preferred for retail users). You are using tools like modoru to downgrade