How to update my iPhone to iOS 13.4

You cannot. This is because iOS version 13.x is no longer available as an update. The current available iOS versions are iOS 15.7 and iOS 16.1.1

NOTE: If you have an iPhone 6, then it can only be updated up to iOS 12.5.6.

If you have an iPhone 6s or an iPhone 7, then those can only be update to iOS version 15.7

If you have an iPhone 8 or newer, then it can by updated to iOS 16.1.1

Which iPhone do you have and which iOS version is currently installed on it?

Axel F.

Depending on your model of iPhone, you might be able to update to either iOS 15.7.1 or iOS 16.1.1.

What model name of iPhone do you have?

If you are not sure......Settings > General > About > Model Name