Latest version of iOS for iPhone 6 plus
Latest version of iOS for iPhone 6plus
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So it does seem that you want to update your iPhone 6 Plus to the latest version of iOS which is iOS 15.1.1
Your device is not compatible for iOS https://www.apple.com/ios/ios-15/check this resource for a full list of device's compatible for iOS 15)
You want to create a full backup of your device before attempting any update > https://support.apple.com/en-us/HT203977
To update your device to the latest version that it is compatible with please follow > https://support.apple.com/en-us/HT204204
Hope this helps ;)