That's basic information on BIOS flashing I wrote a while back.
Find the appropriate BIOS for your board. Find out what flash utility you need based on the maker of your BIOS, (Award, AMI, Phoenix)
Generally your board manufacturer will have a link to a flash utility near the place where you actually downloaded the BIOS.
Read the instructions carefully and have at it.
Be warned though if you screw it up, accidentally power down, have a power failure during the process, or flash with the wrong BIOS you will leave your motherboard completely useless. I even wrote some info regarding BIOS recovery, buuuut since it sounds like you're fairly new to it, I doubt you'd be interesting in hot flashing, so be careful and read up