How to update my iPhone to an iOS 12.6.5

Hello!

If supported, learn how to update your iPhone or iPad to the latest version of iOS or iPadOS: Update your iPhone or iPad - Apple Support.

-Jack

How to update my iPhone to an iOS 12.6.5

Is not not possible.

iOS 12.5.6 is the latest iOS security update for iPhone 5S.

Go to Settings -> General -> Software Update -> Download and Install.

(if you haven't already done that).