If you are wishing to stick with AT&T, you can upgrade your device through them. I would contact them by navigating to their website or into one of their nearest retail stores.
Click here to compare AT&T devices and plans
However, if you didn't want to stick with AT&T, I have created a link to our web page that shows you all of your available options when purchasing a new iPhone! You can manually select which iPhone you wish to upgrade to and see all the different plans and prices.
You may find a better deal than the upgrade with AT&T by purchasing a new device with a new carrier.