How to un-jailbreak or update a jailbroken iPhone 3gs?

I have a 3gs, it was originally on the Vodafone network but I'm Meteor so had it jailbroken. However, I wasn't aware that if I got it jailbroken it was stuck on version 4.1 and no apps are available for download on this version - thus the phone is nearly useless to me. I'm under the impression that if I update it on itunes, the phone will be locked forever from any network and will not be available for use at all as a phone - is this true? If I restore the phone will it revert back to the vodafone network or will it just lock completely?
I'm willing to switch networks if it means I can update the phone but amn't sure if I'm able to do this now that it has been jailbroken.
Is there anyway to either:
a. Reverse a jailbroken phone
or
b.update my software to the latest (version 6.something)
Any help would be greatly appreciated!

I'm not very tech savvy so in simplest terms - I do know that there's a Cydia app on the phone now?

Also I don't have anything on the phone thats worth saving so it doesn't matter to me if I lose apps or anything at all!

Itunes - restore to factory settings