How to add more storage or memory to my iPad

Internal storage cannot be upgraded.

iPad mini also cannot be updated beyond iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular).

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Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12 - and have withdrawn older versions of their Apps from the Apple App Store, now limiting utility and usefulness of older devices.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

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

You can not add more storage to an Ipad, you can add iCloud storage, but that's just for pics and bits, but the physical storage of an iPad can not be increased.

What happens when you try and update to iOS 10?

You can't update to iOS 10, sorry I was looking at another iPad model, as Lotus said you can't update any further