is your PC that you connect your 770 to running Windows, Linux, or Mac OS?
I actually tried mine from both Linux (Ubuntu) and from Windows XP, and found XP much easier...

For windows:

1) download new firmware image
2) install Microsoft .NET framework 2 or greater (if you don't already have it. available from "Windows update" as an optional download. You can see if you already have it installed by looking in the "Add/Remove Programs" list for "Microsoft .NET Framework", and it should have a version next to it.)
2) install Nokia software updater for windows (http://www.nokiausa.com/A4410232)
3) run the Nokia Software Updater, and it will run you through with a set of prompts. At some point you select the firmware image you downloaded in step 1.