How do I update my iPhone 12 to iOS 15?

If this is for your iPhone 12; you can only download and install the “current” iOS …

… which is 17.4.1.

Since you’re several versions out of date, you might need to update using a computer and cable.

Update your iPhone, iPad or iPod touch with the Finder – Apple Support (AU)

You cannot.

The iPhone 12 supports iOS 17, and a such that will be the only version you are offered from Settings ➜ General ➜ Software Update or through iTunes, the Finder or the Devices App.

iOS 15 is no longer signed or available for devices that support a newer iOS version.

Use a computer or the Software Update function in Settings to install 17.4.1. It's not possible to update to an intermediate one for any given model.

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