To reset your email account passwords in DotNetPanel/WebSitePanel (DNP/WSP) first login to your control panel http://yourdomain.com:9001/ (see KB article for more details).
Once logged in hover over the "Mail" icon and click "Accounts".
Click the email address to update.
Enter the new password in the "Password" and "Confirm Password" boxes and click "Save".
You will now need to update your email client to use the new password.