How to update iPhone 6 Plus

The iPhone 6 Plus can be updated to iOS 12.5.5. If the is not the issue, see:

Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

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You don't, sorry. See why in this compatibility chart:

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