Depending on what kind of bios update is available, you have two options.
Option 1: Download a file that creates a boot disk with the BIOS update on it, boot from the disk and it will update as needed.
Option 2: Download a file that updates the BIOS from your windows desktop, this is only on newer machines.
WARNING: DON'T POWER OFF your PC during these updates, it is very serious. You can render you computer USELESS....