you will need a usb stick.min say 250mb (.pup or update file should be around 180mb) on the usb stick make folders like this. PS3 then inside PS3 make a folder called UPDATE must all be capital letters to.
now you will have to decide the cfw you want to install.i personally like rebug 3.55.2 myself but kmeaw 3.55 is good too.its totally up to you.
when you have decided search on ps3news in the cfw section and download the cfw. you really want to get your cfw .pup from somewhere safe and reputable as it could be totally fake or anything if you get it surfing some dodgey site.
OK so you will download a .pup file (you may need to unrar etc) then put the .pup onto the usb stick inside the UPDATE folder then plug the usb stick into your ps3 and goto manually update firmware by usb in settings on xmb. it will install and you will be on 3.55 cfw
any questions just ask, im tired as so sorry if this is hard to follow.. LOL