I want to update my iPhone 6 Plus to iOS 13.4

Hello ~ You can update to iOS 12.5.7 but no further. Click on the links below….

Update your iPhone or iPad - Apple Support

If your iPhone or iPad won't update - Apple Support

If you need a higher version of iOS you will need a newer iPhone. Click below to shop for a new device…

iPhone - Apple Official Site

If you purchase a second hand device please take a moment to read these helpful links by clicking on below to make an informed decision…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

Unfortunately, that is not possible for an iPhone 6 Plus.

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

You can not. That phone can not run anything later than iOS 12.