How do I wipe my Mac and start over

Restore your Mac to factory settings by erasing your hard drive and reinstall macOS. --> How to erase your Intel-based Mac - https://support.apple.com/HT208496

Restart with the Command and R keys held down, use the Disk Utility to erase the internal drive, and install a fresh OS. If it's running 10.6.8 or earlier, insert a compatible install DVD and press C or Option instead of using the recovery system.

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