Updating my old ipad

That is because your iPad is running the most current version that it is capable of running. The hardware in that device is just not capable of supporting what an updated version of OS needs and does not meet the minimum requirements of iOS 10.

You have either have a 2011, iPad 2nd generation, 2012, 3rd generation iPad OR a 2012 1st gen iPad Mini.

These 8 and 9-year old iPads cannot be upgraded beyond iOS 9.3.5/9.3.6 for cellular data models

If you need an iPad that can run the latest and greatest versions of iOS with latest and greatest features, as well as, the most current apps available, then, you need to purchase a new iPad from Apple or a discounted, refurbished iPad from Apple’s Refurbished & Clearance section of Apple’s Website or a new or refurbished iPad from an Apple authorised, third party retail or online reseller OR other trusted sources for new or used Apple products.

Sorry & Best of Luck to You!

The iPadOS and iOS update system only ever offers the most recent available release compatible with your detected hardware. If it says your device is up to date, then there are no newer releases available compatible with that hardware.

It's because your iPad is up to date and can't be updated any further.