How to upgrade iPhone 6plus iOS 12 version to 15
How to upgrade iOS 12 to latest iOS version in iPhone 6 Plus
iPhone 6 Plus, iOS 12
Question marked as
Sorry, but the built in hardware of an iPhone 6plus dos not support any further iOS upgrades.
You'll have to have at least an iPhone 6s or newer to upgrade to iOS 15
iOS 15 and iPadOS 15 are compatible with these devices - Apple Support