Please follow the following steps:
1) Login to Plesk control panel.
2) Click on Subscriptions.
3) Select the corresponding Subscriptions from the list.
4) Click on Websites & Domains > Click Databases.
5) Click on User Tab.
5) Select the corresponding Database Username from the list.
6) Enter the New Databass Username and password details as marked in RED.
7) Click on Check box " Make the user defaults for this database.
8) Click ok.
Information: The database user's db-new-name account was successfully updated.
