original ipad 3G 64gb AT&T

You can't. The Original iPad cannot update past iOS 5.1.1

You may, however, still be able to download compatible versions of Apps you want for it.

In iTunes on your computer, download the App you want a compatible version for using the same Apple ID that the iPad has for the App Store.

For Paid Apps its best to consult with the App maker to make sure there is a compatible version of the App.

With the App Downloaded to iTunes on a computer you can go into The App Store App on your iPad, Tap on the Purchased tab at the bottom, and look for the App there. Tap on it to begin the download. It will likely say its not compatible, but if there is a compatible version it will offer to download it.

Install the latest compatible version of an app on an earlier version of iOS or OS X - Apple Support

For what its worth, my first gen iPad is still working fine.. I've still been ale to get compatible versions of several Apps.

It is however a 5 year old device, and as such its getting to a point where I know it may not be possible to get any more apps for it. ts now mostly used to watch youtube and things like that.

Electronic devices have a limited life span. That's just the nature of the beast.