First of all: Make backup (root your phone and save apps, contacts, /efs directory from the root and others)
Ok. The easiest way to upgrade your Samsung Galaxy S (i9000) is to use Heimdall. =) This is an Open Source project (see the project page).
What is Heimdall?Heimdall is a cross-platform open-source tool suite used to flash firmware (aka ROMs) onto Samsung Galaxy S devices.
Let’s see it step-by-step: