How can I update my ios

The iPhone 6 Plus can be updated to iOS 12.5.6. To do that, 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. 

An iPhone 6 Plus isn't capable of 13 or above. To install 12.5.6, use a computer or the Software Update function in Settings.

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