i bought a ipad with iOS 9.3.5 , my question is how can update to iOS 10 ?

Difficult to say yes or no when you haven't told us which model. I'm guessing no since that is a version that many devices can run but no higher.

Here's how to...

Update the iOS software on your iPhone, iPad, or iPod touch - https://support.apple.com/HT204204 - When you update you will only be given one update selection which will be the highest available iOS version supported by your device. If it says no update is available and you need a higher version you will have to buy newer equipment.

Short answer?

You don't.

Your iPad's internal hardware is too old and underpowered to run iOS 10 and any later versions of iOS.

Sorry.

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!

If it's an iPad 2, iPad 3, or first generation iPad mini, you can't. Otherwise, use the Software Update function in General or iTunes on a computer to update it to 10.3.3, 10.3.4, or 12.4.1.

(174007)

My money is on that any iPad still at iOS 9.3.5 after all these years is either iPad 2, 3 or original Mini and simply cannot be updated further.

 iOS 9.3.5 , my question is how can update to iOS 10

If you have iPad 2, iPad 3 or original iPad Mini, you can't is the answer. If you need iOS 10 or higher, you need a newer iPad which is capable of running iOS 10 or higher.

As has already been stated here, you don't.

Your iPad model is too old.