I have a 6s . How do I update to 13 or over?

Assuming it is actually a 6S Plus, see: Update your iPhone, iPad, or iPod touch - Apple Support

If the device is at iOS 12.5, it is not a 6S Plus.

This article can help you out in updating your iOS: Update your iPhone, iPad, or iPod touch - Apple Support

See:

You could check the model at: Settings - General - About - Model = ???

Or check the version of iOS at: Settings - General - About - Version = ???