How can I download iOS 15.7.9 on my phone

If you have an iPhone 6s Plus (not an ineligible iPhone 6 Plus), and it is still on iOS 12, it might need help from a Mac or PC to get to iOS 15. See the instructions for updating your device manually in:

Update your iPhone or iPad - Apple Support

Nice

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MacTracker shows that the iPhone 6s Plus originally shipped with iOS 9.0. So someone could have gotten one, updated it to iOS 12, and then skipped the subsequent updates to iOS 13, 14, and 15. In that case, that iPhone would be still be eligible for iOS 15.

I do agree that it seems likely that the phone is an iPhone 6 Plus, misidentified as an iPhone 6s Plus; and in that case, iOS 12 would be the end of the line.

If you are on iOS 12, then you have an iPhone 6plus, not an iPhone 6Splus and you phone can't be updated further.

help me how to update my iPhone

I don’t know how update my iPhone

Help me out

Yes

15.7.9upated why