Unlike a standard zip file, psp2updat.pup is encrypted. Sony uses heavy cryptographic protections to prevent users from unpacking the file and modifying the system code. If you were to try and open it with a standard archive manager, you would be met with binary gibberish.
Officially, a user should never need to manually download psp2updat.pup . The PlayStation Vita is designed to update itself automatically via Wi-Fi or by downloading updates directly from a game cartridge. psp2updat.pup