Changing Password for Client login :
Login to plesk panel as an admin user
click on subscription and find the domain name
on the right side of the domain you get the subscriber name, click on the name
select change login info as per below given image
Now regenerate the password for the customer and click on ok button.
Changing Passwords For FTP login :
Log in to your control panel to change the "FTP" Password
Click on FTP link
Select the FTP user from name list and click
To change the FTP password regenerate the password then click ok button.
Changing Passwords for Plesk Users:
Log in to your control panel to change the "Control Panel User Login" for a single domain.
Select your domain name from subscription and click on user tab.
Select the user from the below list
To change user login password click generate button then click ok.