My iPhone 6s Plus won't update

Saying error? what error message do you get?

Are you trying to update your iPhone wirelessly? See this support guide:

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

if you still get no improvements, try to update it via iTunes, connecting it to a computer; see this guide at section "Update your device using iTunes": Update your iPhone, iPad, or iPod touch - Apple Support

If you don't have a computer, borrow one from your friends, or ask for help at an Apple Store or AASP:

Apple – Location Selector

Regards

Giulio

What is the error message you are receiving?

You're most welcome :-)