how do I do a factory reset with my 6s

You can check this out for the two ways to restore your phone. I recommend using iTunes on a computer so that a fresh copy of the iOS software will be installed.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

You don't indicate why you want to reset your iPhone.

First create a backup: About backups for iPhone, iPad, and iPod touch - Apple Support. Then follow the steps listed in: How to erase your iPhone, iPad, or iPod touch - Apple Support.

Finally if you want to use your backup: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support