restore factory settings

You erase the drive and reinstall the OS. That will bring you back to the out-of-the-box condition ready to begin the set up routine. https://support.apple.com/en-us/HT204904

Read the full article as there are two approaches. First just replaces the current OS, second wipes and installs.

Whichever, be sure you have good recent backups. Especially if your erase the disk as erase means erase ... all is gone.

gerrifromchoctaw wrote:

How do I restore my MAC to factory settings?

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