Gigabyte has prepared a BIOS upgrading tutorial:
http://www.gigabyte.com.tw/support/procedure1.htm
Quite complete. Note: replace any Gigabyte entry with your manufacturer's.
BIOS upgrading, step by step.
SOYO offers a great (and simpler, text-based) explanation of:
a) How to flash your BIOS.
b) How to determine if you need a 1Mbit (128KB) or 2Mbit (256KB) BIOS.
http://www.soyousa.com/content/FAQ/47/?n_id=21
Edit: Link updated - Denniss
a) How to flash your BIOS.
b) How to determine if you need a 1Mbit (128KB) or 2Mbit (256KB) BIOS.
http://www.soyousa.com/content/FAQ/47/?n_id=21
Edit: Link updated - Denniss