Restore to factory settings.
I purchased an iMac from my neighbor. How do I prepare this Mac for my use.
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Reboot and hold CMD+R on your keyboard until you see the Apple logo.
Then at the macOS Recovery, click on Disk Utility, click on your Disk Partition (Most likely named Macintosh HD), click erase.
Next, quit DU;
Go to reinstall macOS/OS X, and install.