My phone need to update to iOS 15.0

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device.

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support Read the article for recommended criteria for being able to use wireless updating.

You can also use Mac and PC computers to update system software --> Update iOS on iPhone - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date (and can't go any higher) and won't install anything. If you want to run a higher system version than your current device is capable of running, you need to buy a newer device.

An iPhone 8 or later will only update to the latest iOS, 16.5.1

iOS 15 is not available to your phone.

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]