how upgrade to iOS 12 on Iphone 5s?

Settings->General->About->Model

Post the number here, and we can tell you what iPhone it is.

Also, if it has Touch ID / fingerprint scanner in the home button, its a 5s. If it has a standard home button with a white square, then its a 5 or 5C. If it has a plastic colored back its a 5C.

For more, see here: Identify your iPhone model - Apple Support

5S should take iOS 12

Sure its not a 5C?

You are sure it's a 5s and not a 5 or 5C (those two models only support iOS 10.3.3 / 10.3.4) : Identify your iPhone model - Apple Support ?

Settings, General, About, Model name

See previous two replies

Understood, but what does it indicate for Model.