How do I update my iPhone 6 Plus 12.5.6 to 13.4.1

An iPhone 6 plus can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 _

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

1) Your iPhone 6 Plus with 12.5.6 can only update as far as iOS 12.5.7

2) Even if you could update, you'd get the highest update available for an iPhone 6s to iPhone 7x and that would be at least 15.7.3

3) In order to go from 12.5.6 to 12.5.7 here's two different ways to ensure the highest iOS that your device will support is on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support