I want to update my iPhone

To update to the maximum of iOS 12.5.6, 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 higher. To install 12.5.6, follow the instructions in the Related Article above.

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