Update my iPhone 6 Pluse

If it is an iPhone 6 Plus, as you said the title of your post, you can update it to the latest version of iOS 12. It will never run iOS 13 or higher.

About iOS 12 Updates - Apple Support

If it is an iPhone 6s Plus, as you said in your tag line, you can update it to the latest version of iOS 15. It will never run iOS 16 or higher.

About iOS 15 Updates - Apple Support

The 's' in the name matters – the '6s' models are more recent than the '6' ones, and can be upgraded further.

Update your iPhone or iPad - Apple Support

If this is an iPhone 6s Plus, see:

Updating iOS: common issues and solutions – Apple Community

If this is an iPhone 6, the reason you're not being offered iOS 15 is that your phone is not compatible with it.

Hi there,

The latest available version of iOS for your iPhone 6 Plus is iOS 12.5.7. There are and will not be any more major version updates for for that iPhone.