Try going to Tools & Settings > Database Servers and click the Remove button.
If you are unable to edit the username and password for the database server in Plesk, you can remove the server from Plesk and then create a new database server outside of Plesk. You can then add the new database...
Try using SSH:
- Connect to the Plesk server via SSH
- Run the command: sudo plesk dbm host-delete --hostname=<database server hostname>
- Enter your Plesk password
I am not sure about MySQL, but you can try it:
- Connect to the Plesk database
- Run the query: DELETE FROM data_bases WHERE name...