To change the password of your email address from Plesk control panel:-
1) Log in to Plesk control panel.
2) Click on "Domains" and then Click on "Control panel" in front of the domain whose files permession you want to change.
3) Click on the "Mails".
4) Click on email address for whome you want to change the password.
5) Enter the new password in front of password textbox. You need to enter the password 2 times.
6) Press "OK" once you have entered the password.