ipad 3rd generation

You don’t.

If your 7-1/2-year old iPad 3 is already running iOS 9.3.5/6, then it is already at its maximum iOS version.

If you need an iPad that can run the latest and greatest versions of iOS, as well as, the most current apps available, then, you will need to purchase a new iPad from Apple or a discounted, refurbished iPad from Apple’s Refurbished & Clearance section of Apple’s Website.

Best of Luck to You!

To what version? The iPad 3rd gen was released in March 2012 and is now outdated and incapable of running an iOS version newer than 9.3.5 (for the Wi-Fi model) or 9.3.6 (for the cellular model). It's hardware doesn't support anything newer, sorry.