How to upgrade a jailbroken iPhone 3gs to the most recent ios?

I used to own a jailbroken iPhone 3gs that used cydia but I have recently purchased a BlackBerry and no longer need the jail break.
I want to upgrade my ios to save me having to buy an ipod.
How can I do this?

Just plug the phone into the computer and iTunes will prompt you to update. Then just hit download and install and it will update.