i just want to update my phone to ios 15. how? only ios 16 appear on the general update settings.

You can't. Apple doesn't allow intermediate iOS versions to be installed.

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The option to update will not appear unless the device is qualified to run that iOS version.

For example......an iPhone 7 will never display an option to update to iOS 16.....because it can't run iOS 16.

No it's compatible with all models from 8 on up and all capacities