How to update our iPhone 6s to the new software update?

Hello, that's possible that you have not enough memory space on your phone?

Also try to

-delete already installed iOS update files in memory settings

-charge and do hard restart of your phone

-try to download update again

Wish you luck,

Ara.

See If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support. If you're trying to update over the air try using iTunes or Finder instead, or vice versa.

tt2