You can boot from the install CD/DVD that came with the Mac, there's a reset password tool there. Or if it's a new Mac, from the Recovery partition via Command + R when you restart.
You can also reset a forgotten Mac password without a Mac OS X installer CD/DVD or recovery partiton. This is basically the be-all-end-all approach if nothing else works, because it's guaranteed to get you back into a Mac when the other options are not available, and it works for all versions of OS X. The steps may seem a little intimidating but it's easy if you follow them exactly:
Stage 1) Boot into Single User Mode and remove a setup file
- Restart the Mac holding down the Command+S keys, this will take you into Single User Mode and it's Terminal interface
- You'll need to check the filesystem first, type the following command exactly, followed by the enter key:
- fsck -fy
- Next, you must mount the root drive as writeable so that changes will save, type the following command exactly, followed by the enter key:
- mount -uw /
- Now, type the following command exactly, followed by the enter key:
- rm /var/db/.applesetupdone
- After removing the applesetupdone file, you need to reboot, type 'reboot' and hit enter.
Stage 2) Create a New User Account upon System Boot
The hard part is now over, no more command line. In this step we just create a new user account as if you just got a new Mac:
- Upon reboot, you will be presented with the traditional "Welcome Wizard" startup screen just like when you first get a Mac
- Follow the welcome wizard and create a new user account - making the account name different from the account whose password you want to recover, make this new user account is an Administrator
- Continue on and boot into Mac OS X with this newly created user account
Stage 3) Reset the Forgot Password via System Preferences
Now you just need to reset the forgotten user account password using the Accounts control panel:
- Once you are booted into Mac OS X, click on the Apple logo and then navigate down to "System Preferences"
- Click on the "Accounts" icon in System Preferences
- Click on the Lock icon in the lower left corner of the "Accounts" preference window and enter the newly created user credentials, this enables you to change other user accounts and reset other users passwords
- On the left side user panel, select the user account containing the forgotten password
- With the user of the forgotten password account selected, click on the "Reset Password" button
- Enter a new password for that user, be sure to include a meaningful hint so you don't forget it again!
- Close System Preferences and reboot the Mac
- You can now login to the previously inaccessible user account using the newly reset password! All user files and settings are maintained as before the password was forgotten
Now you can delete the temporary account you created to reset the users password. This is probably wise for security purposes.