How do I update my iPhone without Wi-Fi or iTunes

It’s either Wifi or iTunes. The wifi method is found in General > Software Update.

If your device is quite old; it may be located somewhere else and may not be compatible with the latest version but should allow you to download the last compatible version, unless however it’s an extremely old (iPhone3 or earlier) that it can’t download the most recent and compatible update.

Short answer: You don't. You must use either iTunes on a computer or you need access to Wi-Fi. Use a Public Wi-Fi instead.