How to update my iPhone 6 iOS 12.5 to iOS 13.4

You have an iPhone 6. You cannot update the iPhone 6 beyond iOS version 12.5.6.

You will need a newer model iPhone to update to iOS 15.x.x.

Axel F.

You can't, You need a newer phone.

iOS 12.5.6 is the current end of road for your device.