How to update a software
How do I update an iPhone6 plus
iPhone 6 Plus, iOS 12
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Your iPhone 6 Plus can only update as far as iOS 12.5.7
Here are 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