upgrade to iOS 13.3
I have used 10.6 go out of 16 gb but still unable o upgrade to iOS 13.3..how can I upgrade it??
Are you sure it's a 6s plus? (Settings>General>About>Model... then post your model here)
I'm asking this because if it is a 6 plus (not 6s plus) you can not update it to iOS 13 or later.
If your sure it's a 6s plus, see this support guide:--> Get help with over-the-air iOS updates – Apple Support
if the issue still persists, update it via iTunes on a computer. If you don't have a computer, borrow one from your friends, or ask for help at an Apple Store or Authorized Service Provider. They will help you.
Regards
Giulio