How to download iOS 15

If you have an iPhone X, you can only update to the latest version, which right now is: iOS 16.3.1

If your iPhone X is on an older version and you are not offered to update directly to iOS 16, then you must update your iPhone from a computer. See the links at the bottom of my answer.

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

Remember to connect to Wi-Fi and a charger. Your iPhone must have more than 50% power to update without a charger connected.

You can find more useful information here:

Update your iPhone or iPad - Apple Support

If you want to update your iPhone via a PC or Mac, see here:

• Mac: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
• PC (iTunes): Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

You cannot update to an intermediate version. To install iOS 16.3.1, go to Settings -> General -> Software Update and install the particular update.