Want to update my iOS

To update to the most recent, compatible version of iOS 12.5.7, please 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; they require at least 2GB of RAM. To install 12.5.7, use a computer or the Software Update function in Settings.

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