To change a password on a POP3 account, simply login to your Control Panel. Click on the "Mail" link and then "Manage/Add/Remove Accounts". Now, click the "change pass" to the right of the email account you wish to change the password for. Type the new password in the fields provided and click "Save".
Your new password for this POP3 account will be needed the next time you try to check your e-mail on this POP3 e-mail account!