Forgot Your Account Password
For Lion or later
Reset a Mac OS X 10.7 Lion Password
OS X Mountain Lion- Reset a login password,
OS X Mavericks- Solve password problems,
OS X Yosemite- Reset a login password.
macOS Sierra-Reset a login password.
Change or reset the password of a macOS user account - Apple Support.
For Snow Leopard and earlier with installer DVD
Mac OS X 10.6- If you forget your administrator password,
OS X- Changing or resetting an account password (Snow Leopard and earlier).
For Snow Leopard and earlier without installer DVD
How to reset your Mac OS X password without an installer disc | MacYourself
Reset OS X Password Without an OS X CD — Tech News and Analysis