iPhone update to new iOS

There is no Apple device capable of updating to iOS 13.4. If you could tell us which device you have, we may be able to offer assistance.

As Tesserax points out, you can not update to iOS 13.

That model can be updated to iOS 12.5.7, no further.

You didn't provide us with the exact model of your iPhone, but if it is an iPhone 6 or older, it cannot be upgraded to iOS 13.x.