You can change the password of your mailbox by following these steps:

1. Open your web hosting Control Panel's Mail Manager section.
2. Click on the Password button for the mailbox in question.
3. Enter a new password in the New Password text field.

You will notice that there is a password strength indicator below the New Password field. It will update in real time as you are entering your new password. There are five distinct levels of password strength: Very WeakWeakFairStrong, and Very Strong. Along with the password strength indicator, you will also get information and hints in a tooltip beside the indicator. 

Change mail account password

Note that the Submit button will remain inactive (grayed out) if the password level is Very Weak or Weak. You will only be able to set a password that is deemed FairStrong, or Very Strong. You can check the following article for tips on choosing a secure password:

Online Manual » Security » Choosing a password
You can also utilize the random password generator:

Online Manual » Control Panel » Random password generator

Change mail account password 

4. Re-type the new password at the Confirm Password field.
5. Click the Submit button.