Downloading iOS 15.0
I don’t have a laptop or tablet so how do I download a 15,0 To my phone I iPhone 8
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You can update your iPhone wirelessly, have a look at the steps in this article:
Update your iPhone or iPad - Apple Support
But, you can not choose iOS 15 as an option. Your iPhone will be upgraded to the latest version of iOS 16, as this is the highest supported iOS version for an iPhone 8.