How do I backup my iPhones 6s before passcode is forgotten

I’m not quite sure what you’re asking about, here.

If you’ve forgotten the passcode, there’s no way to preserve the contents of the device, other than a backup made prior to forgetting the passcode.

How to back up your iPhone, iPad, and iPod touch - Apple Support

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Eatapplesxtz wrote:

How to backup iPhone 6s

How to back up your iPhone: How to back up your iPhone, iPad, and iPod touch - Apple Support