how to I install Mojave from High Sierra

Back up your data, check your applications for compatibility, and if your Mac is a 2012 or newer model, or a 2010 desktop Mac Pr upgraded with a Metal-capable graphics card, click on Mojave. Otherwise, you can't.

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Open System Preferences and click Software update. If it says

Another Update is Available. More Info

Click on More Info.

And there it is