i am not able to update my i pad mini to latest version from 9.3.5

iPad mini is prolly no longer supported. IOS10 and above require 64bit chips and the mini does not have that.

you prolly have ipad mini gen 1

Support ios11

• 12.9-inch iPad Pro
  2nd generation
• 12.9-inch iPad Pro
  1st generation
• 10.5-inch iPad Pro
• 9.7-inch iPad Pro
• iPad Air 2
• iPad Air
• iPad
  5th generation
• iPad mini 4
• iPad mini 3
• iPad mini 2

If you have the original iPad Mini, you've likely gone as far as you can go. This link will tell what iOS version will run on which iDevices by model: Maximum iOS Version for iPod, iPhone & iPad: Everyi.com