I want to update my iPhone 6 to 15 iOS

Unfortunately, the iPhone 6 is not compatible with iOS 13 or higher.

That model can be updated to iOS 12.5.7, no further.

While you seem to have already solved your tech support issue, you will find my reply is 100% accurate.

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If it is a model 6, you don't, sorry, since it cannot support iOS versions past 12.5.7.

If it is a model 8 (as per your product list), you'll get the latest iOS version (16.4.1 or 16.4.1(a).

Take a look at this compatibility chart:

I want to update my iPhone 6 to 15 iOS

Please see the replies above.