How to update an iPad from 2015 to iOS 10? It currently has iOS 9.3.5 and says- up to date.

If its saying 9.3.5 is up to date in Settings->General->Software Update, then its unlikely to be a 2015 iPad at all.

You may have bought it in 2015, but its more likely a 2011 or early 2012 model.

2 iPads were released in 2015. The iPad mini 4 and the iPad Pro 12.9" first generation. Both can update directly to iOS 13.

Unless your iPad is an iPad 4 (2012) or later, it cannot be updated beyond iOS 9.3.5

See here: Identify your iPad model - Apple Support

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. The 32-bit architecture upon which they are based is effectively obsolete; most later versions of iOS are designed for 64-bit system hardware.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store, now limiting utility and usefulness of older devices.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

IOSupgradehelp wrote:

How to update an ipad (2015) to iOS 10?

You don't. Your ipad is at the end of the line for updating ios.

You will need to purchase a newer ipad in order to get the latest ios.