Your BIOS update should be available at the manufacturers wesite for free. If it's a self built system then checkout
www.kobian.com (The site won't open for me right now) If it's a retail PC (Gateway, HP, etc) then go to that website and enter the PC model in the support section, and look for downloads, or post the computer make and model number here and we can help you find the BIOS download.
May I ask the reason you want to update the BIOS. It's usually only needed if you are adding hardware that is not recognized by your current BIOS, such as a new CPU.