1. Load your Plesk Control Panel.
    This can normally be found at https://www.youdomain.com:8443 if you did not know already.

2. Login to the Plesk Control Panel with your username and password

3. Click the 'Mail' button once you have logged in.

4. On the mail page, you should see a list of current setup users, like so:

5. Click on the username which you want to change the password of.

6. You should then see this screen:

7. Enter the new password for the mailbox in the boxes similar to those shown above

8. Scroll down to the bottom of the page and click 'Update'

You have now hopefully changed your mailbox password.