Since your WebMail account is actually your POP email account, changing the password of your POP email account should do the trick. At any given time, you can change your POP email account's password by following these steps:
1. In your control panel (cPanel), click on the "Email Accounts" link under "Mail".
2. Click the small padlock icon corresponding to the particular POP email account that you want to change the password for.
3. Enter your new password in the "New password" field and click "Submit".
Note: If the password was changed, the following will appear (except name@yourdomain.com will be the email account that has its password changed):
The changes for the e-mail account were changed.
name+yourdomain.com --> new password
