Well, first you need to determine which OS you are currently running-you can determine that by going to Options>About.
Next, find the link to download the OS version that matches the one on your device...the sticky at the top of this section will get you started:
http://www.blackberryforums.com/gene...n-devices.html
If you have an older OS version, there is a link at the bottom of the post titled "Previous OS Here"...keep clicking this link on each previous OS post until you find yours.
Once you have downloaded the OS to your PC , it can be found (if you are using Windows) here: Computer>Program Files>Common Files>Research In Motion.
Once you ave gotten that far, post here again, we'll pick up from there.