How to update jailbroken iPhone?

I decided to downgrade and jailbreak my iPhone 4 6.1.3 to 5.1.1 and I've had a right fuss with it all and I've finally decided I'd like to go back to how it was. How would I do this?

I had to use a custom ipsw to downgrade so would I just create a new custom ipsw with the 6.1.3 blobbs and then restore that or can I just restore using the stock firmware in iTunes?

Www.jailbreakme.com/ & good luck not BRICKING your Iphone [99% chance of brick with a downgrade]