Yeah he's right, your best bet would be check out the manafacturers site, and they will have downloads that you copy to a bootable floppy and you upgrade in dos, but only do it if you know what your doing, and if theres any chance of a power failure dont! If the power goes mid way, your bios is basically frozen!
But there should be a step by step thing on the site, iv updated a few in my time, so if ya have any questions just pm me or post up here!
Why are you upgrading if you dont mind me asking!?