The downloaded zip-file should contain a readme-file with a step-by-step description. There are different ways to update the BIOS, depending on the hardware (i.e. Mainboard, drives) and software (i.e. operating system) you are using.
how do i know when it`s the right one
When the system boots up after the update-procedure, it must have been the right one :-)
You need to know the actual manufacturer of your mainboard. Furthermore, the exact model-name and the revision of the mainboard. You can find these informations printed on the board.
With these informations you search the Support-section of the manufacturer's website. Download the latest BIOS available and you're done.