Completely wipe computer
How do I wipe everything off of my Mac book
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There are a few ways to erase and re-install a new macOS in your Mac.
• How to reinstall macOS from macOS Recovery - Apple Support
https://support.apple.com/en-us/HT204904
"Learn how to use macOS Recovery to reinstall the Mac operating system on your built-in drive
or an external drive. Reinstall from macOS Recovery macOS Recovery makes it easy to reinstall
the Mac operating system, even if you need to erase your startup disk first."
There are other instructions in Support articles, these vary somewhat.
Good luck & happy trails!🌞