OS System

Restart with the Command and R keys held down. If your computer's firmware or model is too old for that method to work, you need to use a thumbdrive or DVD.

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Another thing to keep in mind is that if it works to restart the computer using the Command + R keys, the computer will download the original operating system that it came with. If you were using a newer OS, then you will have to then download and install the newer OS. I hope it goes well for you.