Accidentally formatted High Sierra operating system, how to reinstall? A family member has accidentally formatted her High Sierra operating system on her Apple computer. Over the phone, she asked me how to download and reinstall the operating system again. I haven't much experience with Apple ecosystem, can anybody suggest a solution? Sorry if this question looks like a stupid one.
Try using the build in Restore. 

As the computer restarts, hold down the Command (⌘) – Option (⌥) – R combination immediately upon hearing the startup chime.

Follow the instructions on the screen.