How to get iOS 13 for my iPhone 6?

You cannot. The iPhone 6 does not have the right hardware to run iOS version 13/14/15 efficiently. The highest that you can update your iPhone 6 is iOS version 12.5.5. You need a newer iPhone to update to higher versions.

Axel F.

iOS 12 is the highest iOS version you can get for an iPhone 6.

See:

iOS and iPadOS devices listed by the last compatible OS

Sorry, but that is not possible.