Need help updating iOS 10.2.1 to iOS 11

You are going to have to tell us what the problem is before anyone here can help. We cannot see your device. What does "it doesn't let me" mean? And since you indicate that wireless downloading isn't an option, are you indicating that you are attempting to update via iTunes?

GB

There could be many reasons why you can't update your device wirelessly: most common are "not enough space on your iPhone to manage the update" or "wifi network unreliable / unstable".

Read carefully ALL the steps provided in this support guide:

Get help with over-the-air iOS updates - Apple Support

If you stiil can't update your iP6s wirelessly, you need to do it via computer using the latest iTunes version (now 12.8).

If you don't have a computer (or your computer doesn't work), borrow one from your friends, or contact your most convenient Apple Store or Authorized Service Provider, explain your problem, and they will help you to perform the update operation:

Apple - Find Locations - Location Selector

Regards

Giulio

What does it tell you when you try to update?

Richard