How to skip older version to update to iOS 15

Provided that your device is capable of supporting iOS 15 (see here) it should be offered as a download/install in Settings > General > Software Update.

I recall that at the foot of that screen on my iPhone was the invitation for iOS 15, while at the top was the status of the currently installed version of iOS - advice that an update was available or notification that it was up to date.

Hi there! Could you provide your IOS version and iPhone Model? This would help me understand a little better what you are trying to do. To find the software version installed on your device, go to Settings > General, then tap About.