I want to know how to upgrade a jailbroken iPhone's firmware to 2.0 or 3.0 from version 1.0

The iPhone OS 3.0 is available only to developers at the moment. It won't be released to the general public until this summer (Apple hasn't revealed an exact date). Unless you're really, really desperate for it, I would hold off installing it officially launches. In its beta form, 3.0 is a bit buggy, and it slows down the iPhone by quite a bit.