how do i update from ios 9.3.5

You can't.

You own an old 8 or over 9-year old iPad that cannot be upgraded any farther than iOS 9.3.5/9.3.6 for cellular data models.

Here's why and what you will need to do about it in 2020!

Tap and read my link below for my additional info about your old iPad model.

If you own OLD 8 and 9 Year Old iPad mo… - Apple Community

These old 8 and 9-year old iPads will very, VERY SOON be fully obsolete by the end 2020.

These old iPads are DONE!

Sorry & Best of Luck to You!

You clearly have an iPad2, iPad3 or original iPad mini. These devices cannot be updated beyond iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular).

Apple ended update support for these devices in September 2016. None of these models can be updated to iOS 10 or later versions, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store, now limiting utility and usefulness of older devices. 

If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad7 model supports current iPadOS 13.x - and has hardware specifications and capabilities that will vastly outperform your old iPad

I hope this information is helpful - if only in clearly stating your current options.

If your ipad is stuck at 9.3.5 then it probably cannot be updated. THere are a few models of iPads (the 2nd gen for example) that max out at 9.3.5.