If the computer in question doesn't have a CD drive, you can either hold 'Option' at boot and choose the 'Recovery partition' at startup, or put the Mountain Lion or Mavericks installer on a flash drive. It will boot into the Mac OS X installer. Just insert the CD into the target Mac and hold the 'c' key as you boot up the computer.

If you have the Mac OS X installer CD handy, it's super easy to change the administrator account's password. Reset Mac Password with the OS X Installer Using and enabling FileVault for disk encryption can also be beneficial to users who are generally concerned about data security and privacy.Method 1. If the ease of this concerns you at all, you can make things more secure and prevent the easy password resetting by setting a firmware password on the Mac, but do be forewarned that if you forget the firmware password it can be a much more significant ordeal. While this is fairly advanced, it’s also relatively easy. That’s all there is to it, the password will have been reset to MacOS for the user account you chose.

Choose to “Restart” the Mac and when the Mac boots up, use the newly reset password to login to the computer.Enter a new password, confirm the new password, set a password hint (recommended) and then click on “Next” to set the new password for the account in question.Hit the return key to launch the Reset Password tool, then choose the user account or admin account you want to reset the password for.When Terminal loads, type the following exactly:.At the “MacOS Utilities” screen, pull down the “Utilities” menu and choose “Terminal”.Reboot the Mac, as soon as the screen turns back on or you hear the boot chime sound, start holding down COMMAND + R keys concurrently to boot into Recovery mode.This is the simplest way to reset a password on a MacOS Sierra computer: For earlier MacOS versions you can also follow these tips for handling a forgotten Mac password meaning you’re not out of luck if you don’t have Sierra or a modern Mac OS release. Before beginning, keep in mind that internet connected Macs running Sierra and other prior versions of Mac OS can also reset a password by using an Apple ID after incorrectly entering a password a few times, which may be a better approach for some users.

To be clear, this allows you to reset any password for any user on a MacOS Sierra computer, including for the admin account.

Imac admin password reset