I want to update my iPhone6 to iOS 13.4

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

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

iPhones 6, 6 Plus, and earlier, can only run up to iOS 12.5.7

iPhone 6 can be updated as far as iOS 12.5.7. No more updates after that.