How to update iPhone 6 Plus

To update to the most recent, compatible version, 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. 

Please note that iOS 13 or higher is not supported for the iPhone 6 Plus. The latest iOS version for this model is 12.5.7.

An iPhone 6s Plus can use up to iOS 15.8.1.