Ensure your PS3 model is compatible. Early "Fat" models and some "Slim" models (20xx and early 25xx) support full CFW. Newer "Super Slims" usually require PS3HEN instead of a full PUP swap.
You can never install a firmware version lower than what your console shipped with from the factory. The Verdict firmware ps3updatpup exclusive
For a PS3 to "see" this file on a USB drive, it must be placed in a specific folder hierarchy: USB Drive > PS3 > UPDATE > PS3UPDAT.PUP Official vs. Custom Firmware (CFW) Ensure your PS3 model is compatible