If you are referring to the administrator password, then there are two ways to reset it.
If you have the OS X installation disc, then follow the steps below to reset the administrator password ***** the Mac OS X installation disc.
• Insert the Mac OS X installation disc 1 to your Mac.
• Turn off your Mac
• Turn it back on and immediately hold down the C key on your Keyboard for 20 seconds.
• This should boot your Mac directly into Mac OS X installation screen
• Choose your Language and click Next button. Don't click Next again.
• Choose Reset Password ***** the Utilities menu (from the top bar)
• Select your Mac OS X hard disk volume. Select the user name of your administrator account to reset the password.
• Enter a new password ***** Click Save.
• Restart your Mac. Use new password.
If you don't have the OS X disc, then you can create a new user account from Single User Mode. Then login to that new account and you can change the password ***** your old account.
Follow the steps below to create a new User Account in Single User Mode.
• Reboot your Mac
• Immediately hold down the Command(Apple) and S keys together for 10 seconds to boot into Single User Mode
• When you see the text prompt, type in the following terminal commands to create a brand new Administrator account (hitting return after each line). Also, note down the spaces between the commands, like a space between mount and -, space between -uw and / etc
mount -uw /
shutdown -h now
After rebooting, you should be able to create a brand new Administrator account. Then go to System Preferences > Accounts, select your old account and reset the password (you may need to unlock that screen by pressing the lock button at the left corner to make changes). Then logout from Apple menu and login to the old account with the password ***** reset.
If you get any message about keychain after resetting the password ***** logging in, then follow the steps below to solve it.
Open Keychain Access, which is in the Utilities folder within the Applications folder.
From the Keychain Access menu, choose Preferences.
Click General, then click Reset My Default Keychain.
Authenticate with your account login password (the password ***** reset earlier).
Quit Keychain Access.
If you have any difficulty on following the steps above, please let me know and I will walk you through it.
If this is not the admin password ***** want to change, please let me know by replying in here and I will continue to help you.