I want to update my iPhone to 13.0

No. If your iPhone is capable of running iOS 13, it is also capable of running iOS 14 so only the very latest iOS 14 would appear as an available update.

If your device is running iOS 12 or an older release and software update says your device is up to date, then it is too old to run iOS 14, hence also too old to run iOS 13.

Basically, iOS 13 is not available any longer. Devices capable of running it can only update to the latest release of iOS 14. Devices not capable of running iOS 14 couldn’t run iOS 13 either even if it was still available.

No. If it's an iPhone 6, iPhone 6 Plus, or older, it doesn't have enough RAM; later models can only be updated directly to 14.5.1.

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