i have an older ipad that is version iOS 9.3.5. how can I update?

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Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store, now limiting utility and usefulness of older devices.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. Of current models, even the entry-level iPad7/8 model supports current iPadOS 14 - and has hardware specifications and capabilities that will vastly outperform your old iPad

I hope this information is helpful - if only in clearly stating the limitations of your current iPad and your available options.

That's an iPad 2nd generation model and unfortunately, due to hardware incompatibility, it can't support iOS versions beyond 9.3.5.

Regards

Giulio

Identify your iPad model, posting here the code that you find on the back of your device (it's an "Axxxx" code, where xxxx are 4 numbers).

Regards

Giulio