Open the main User Accounts control panel. Click on Manage another account and provide administrative credentials. Click on the account you wish to modify. Next, click on Remove the password and then Remove Password to confirm. Note that if there isn't a password stored for this account, you will not get this option. Now, click on Create a password, enter a password of your choice and enter it in the appropriate boxes. Add a hint if you wish, although this is optional. Click on Create password to confirm and then close the dialogue panel.