Select Menu system Preferences → general → transfer or reset → delete all settings and content
Click 'Transfer or reset' on the right-hand side, then click 'Delete all settings and content'
delete all settings and content
Enter your admin information in the deletion wizard and click on “unlock”
Check items that will be removed in addition to your content and settings.
If your Mac has multiple user accounts, click on the arrow next to your account name to check the items.
Click “continue” and follow the on-screen instructions.