How to upgrade 3GS from 4.1 to 4.2.1?

[mklo]

I have a 3GS with 4.1. I would like to got to 4.2.1. I downloaded the 4.2.1 firmware, when I hit shift-restore 4.2.1 itune gave me an error messaage. Can I use other software to upgrade? Thanks.

 

[eastercat]

You need the blob for the ios. If you don't have it, you can only install the most current ios.