If your on 3.55 and you wish to stay at 3.55 then use the 3.55.3 REX pup. can be found at
If your on 3.55 and you want to go to 4.21 then use the 4.21 REX pup. can be found at
Now once you have downloaded the firmware that you wish you will need to rename it like this.
REBUG_3.55.3_REX_PS3UPDAT.PUP to PS3UPDAT.PUP
REBUG_4.21.1_REX_PS3UPDAT.PUP to PS3UPDAT.PUP
Now you will need a usb stick formated to fat 32, then you will need to create a folder on the root of the usb stick called "PS3" then inside that folder you will create another folder named "UPDATE". So it will look like this.
PS3UPDAT.PUP being the firmware that you are choosing to install.