iPad won't upgrade

That model corresponds to a First Generation iPad. Mini.

A first generation iPad Mini cannot update beyond iOS 9.3.5. iOS 9.3.5 is the last update it can get.

What iOS version is it currently running? Settings->General->About->Version?

If its less than 9.3.5 and it won't update to it. you may need to connect it to iTunes on a computer to update.

Update your iPhone, iPad, or iPod touch - Apple Support

Why not? What happens when you try?

What model of iPad do you have exactly?

If it can run a later iOS version than 9.3.5 then it won't download 9.3.5 and instead download whatever iOS version is the most up to date version it can update to instead.