Software Update

iOS is for iPhones, iPod touches it does not work in Macs.

Macs run macOS.

A 2015 MacBook Air can update all the way to macOS 11 Big Sur. link-> How to upgrade to macOS Big Sur – Apple Support

If you have a specific reason to want macOS 10.14 Mojave, see here: link-> How to get old versions of macOS - Apple Support

Back up your data, check your applications for compatibility, and click on Mojave.

For reference, Macs run macOS, not iOS.

(200210)