First, log into your Plesk control panel and then click on Domains from the main menu.
Click on the domain that the database user belongs to.
Now click on the Websites & Domains tab at the top of the page.
You should see a selection of icons now in the middle of the page with large descriptions, click the Databases option.
Click on the name of the database that the user you would like to modify belongs to.
Now click on the user that you would like to modify.
From this page you can modify the username, password or both for this MySQL user account. Click Ok once you are happy with your changes to save them.