How to reset iMac's forgotten password

If you forgot your Mac login password - Apple Support describes what you need to do, including the steps to follow If you can't reset your Mac login password - Apple Support.

Follow all of them, all the way through, all the way to and including the end.

If you forgot your login password -Apple Support

What help do you want, D.I. Johnson provided a link to step-by-step instructions. Please click the link and follow the instructions. If you cannot do that then you can either call AppleCare or take the computer to your local Apple Authorized Service Provider to ask for assistance.

AppleCare Contact Info

Apple Authorized Service Provider Locator (AASP) & Genius Appointment