how do I factory reset my mac mini?

Clickhereand follow the instructions. If the computer originally shipped with Mac OS X 10.6.8 or earlier, when you reach step 5, insert its original disk, restart with the C or Option key held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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the concept is technically returning your mac to factory state:

What to do before selling or giving away your Mac - Apple Support

Do a backup before reinstalling, preferable 2 backups on 2 different drives.