To change your password in Mac OS 10.3 or 10.4, follow these steps.

1. Log in with an administrator account. Tip: If you don't know the password of any administrator accounts, see "Resetting the original administrator account password" below.
2. From the Apple menu choose System Preferences.
3. From the View menu choose Accounts.
4. Click the lock button if it appears locked.
5. Select the username whose password you want to change.
6. Click the Reset Password button (Mac OS X 10.4 only).
7. Enter a new password in both the Password and Verify fields, and hint.
8. Click the Reset Password button (Mac OS X 10.4 only).
9. If a dialog box appears with the message "Your Keychain password will be changed to your new account password," click OK.

Source: docs.info.apple.com