how to get 10.14 system update

Check the compatibility, and download the appropriate installer files by clicking the link: How to get old versions of macOS – Apple Support (UK)

Back up your data, check your applications for compatibility, and if your MacBook Air is a 2012 or newer model, click on Mojave.

To check the programs, click here and look them up.

(212486)

Choose Applications from the Finder's Go menu, throw the existing installer away, and try downloading it again.

(212501)