Where do I locate the iOS 13 install

Settings, general, software update or through iTunes on your computer. See here https://support.apple.com/en-us/HT204204

Update your iPhone, iPad, or iPod touch - https://support.apple.com/HT204204 - When you update you will only be given one update selection which will be the highest available iOS version supported by your device. If you do not see an update then you may be mistaken as to the exact model iPhone you have.

Settings > General > Software Update.

But if have been there and you aren't offered the option to update, and assuming you really have an iPhone 6s if you haven't updated over the last several updates, you'll find better success updating using your computer in iTunes.

Use this link: Update your iPhone, iPad, or iPod touch - Apple Support

In the Settings App - General - Software Update