Thanks for asking the Apple Support Communities for help with updating your iPhone 6s. I am happy to offer some options to assist you with this.
First, if you are trying to update your device wirelessly and experiencing issues, this help article recommends using iTunes to try to get the update: Update your iPhone, iPad, or iPod touch - Apple Support
Update your device using iTunes
If you can’t update wirelessly on your iOS device, you can update manually using iTunes on a computer that you trust. If your computer is using Personal Hotspot on the iOS device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.
If you receive any alerts or error messages, the article above links to this second help article stating:
If you see error messages in iTunes or need more space to update
Get help if you see error messages while updating your device with iTunes. Or if you need more space for an update, you can delete content from your device manually.
If you still have issues or are still unable to update your iPhone, please try the troubleshooting in this help article: If you can't update or restore your iPhone, iPad, or iPod touch
Thanks and have a wonderful evening