To change password of an e-mail ID follow the steps mentioned below.
1. Login to Plesk control panel
2. Click the "Mail" Tab 
3. In the next page that opens up, click the e-mail ID for which you need to change password.
4. In the page that opens, you can find two text fileds named "Password" and "Confirm password". Enter new password in those fields. Then click "OK" button.
Now you will be able to login with new password.
