how do i update 10.13.6 to 10.14

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

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I clicked the download for Mojave and got this message: This copy of the install macOS Mojave application is damaged, and can't be used…