factory reset macOS High Sierra

I'd download & make bootable 10.13.6 Installer on a USB stick first...

Create a bootable installer for macOS - Apple Support

Then...

What to do before you sell, give away, or trade in your Mac - Apple Support

jbouj1 Said:

"factory reset macOS High Sierra"

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Reinstalling the macOS Anew:

Try This:

1. First, back up your Mac, then

2. Then, install the macOS anew

In Depth:

1. Back up your Mac:

Start by Creating a Time Machine Backup of your Mac, so that you can have something to restore your Mac's data from, should anything go wrong with the reinstall.

2. Use a USB Bootable Installer:

Create a USB Bootable Installer for Big Sur. Delete the hard drive's partition. Then, install this macOS with. Go Here: How to Create a Bootable Installer for macOS - Apple Support.