Download the PS3 system software from https://www.playstation.com/en-gb/get-help/ps3-system-software/
Make a folder called "PS3" on a FAT32 formatted USB drive.
Create a folder called "UPDATE" in the "PS3" folder.
Put the file "PS3UPDAT.PUP" in the "UPDATE" folder on the USB.
Plug the USB drive in to the PS3.
Hold the power button on the PS3 until the unit beeps three times, then let go.
Hold the power button again until you hear 2 slow beeps then 2 quick beeps, then let go again.
You should now get instructions on the screen and be able to update the PS3 from the menu.