Mini iPad 2

If your iPad Mini has been stuck on iOS 9.3.5, after all of these years, you own a 1st gen iPad Mini NOT an 2nd gen iPad Mini.

A 2nd gen iPad Mini would have advanced to all of the other major iOS upgrades and updates for the past 3 years.

You are sure it's a Mini 2 : Identify your iPad model - Apple Support ?

If it is you can update it to the current iOS version (iOS 12.3.1 ; you can't update to iOS 10) via Settings > General > Software Update.

If it's an iPad Mini 1 you can't update it any higher.