You can create a new password. Or you can simply erase the hard drive and reinstall the operating system.
Forgot Your Account Password
For Snow Leopard and earlier
For Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
When the menubar appears select Terminal from the Utilities menu.
Enter resetpassword at the prompt and press RETURN. Follow
instructions in the dialog window that will appear.
Or see Reset a Mac OS X 10.7 Lion Password and
If you are unable to boot from an installer DVD:
(these will only work on Leopard or earlier)
1. Boot to Safe Mode by restarting and after the chime press and hold down
the COMMAND-S keys until a black screen with white type appears.
2. At the prompt, type the following commands pressing return after each
If you do not receive a "Filesystem OK" message then repeat this command
until you do. If after seven tries you still do not receive a "Filesystem OK"
message, then the system is corrupted and needs to be reinstalled.
mount -uw /
mv local.nidb local.old
shutdown -r now
The second-to-last command above will cause OS X to think that the operating system is newly installed, and when the new owner starts up the computer it will send him/her to the startup wizard where he/she can start a new user without reinstalling.
This last method does not actually change a password. It simply removes the user accounts and lets you create an initial admin account as if the computer were new out of the box.
Reset Admin Password Using The Terminal
You can reset the password by printing out (or writing down) and carefully following these steps.
First, find the short name of the admin user on the machine. The easiest way of doing this is by looking at what the directory is called in the Users folder.
Now, start in Single-user Mode. It should boot into a command line.
Enter these exact lines. Press RETURN after each.
mount -uw /
ifconfig lo0 up
netinfod -s local
Now you'll need that short name. Enter "passwd", a space, and the short name of the admin. For example, if the admin was rather prosaically called "imadmin", you would enter "passwd imadmin".
The computer will now prompt you to change the password for "lmadmin" (or whatever the short name was). Go ahead and enter a new password.
and the machine will reboot. The admin password should now be changed.