To change the password of your email address from Plesk control panel:-
1) Log in to Plesk control panel.
2) Click on "Domains".
3) Click on the name of the "domain" under which email address exists.
4) Click on Mail accounts.
5) Click on email address for which you want to change the password.
6) Click on "Preferences".
7) Insert your new password (you need to insert the password for 2 times) and click "OK". We recommend you to keep your password strong (Do not keep dictionery words).