Why is my iPad mini not updating from iOS 9.3.5

It would seem that you have a first generation iPad Mini. That device is not capable of updating past iOS 9.3.6. The device is 8.5 years old, and well past its prime. Time for a replacement, especially if you need something running the newer iOS.

You can use this to compare models, https://www.apple.com/ipad/compare/, and then see what is available. If you do not wish to purchase new, see what refurbished models are available from Apple. Refurbished iPad - Apple. Unfortunately, right now they don't seem to have any right now. It would really depend on how much you wish to pay, especially if its use is for watching video.