restore my mac os
how do i restore my imac os from the internet
iMac 27″, macOS 10.13
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1. Start up from macOS Recovery
To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later.
Command (⌘)-R Install the latest macOS that was installed on your Mac.
Option-⌘-R Upgrade to the latest macOS compatible with your Mac.
Shift-Option-⌘-R Install the macOS that came with your Mac, or the closest version still available.
Release the keys when you see the Apple logo, a spinning globe, or a prompt for a firmware password. When you see the Utilities window, you've started up from macOS Recovery