Upgrade my ios 9.3.5

If it's a first generation iPad mini, you can't. Otherwise, use its Software Update function or update it from iTunes on a computer.

Gail: It's not possible to tell what model a device is just from its iOS version, unless it's 1.0 or 6.1.6; it may not have been updated in a while.

(163904)

If your device is running iOS 9.3.5, then it is either a 2nd or 3rd generation iPad, and it is running the highest iOS that it can run.

Best,

GB