You can only change the username in the Plesk Control Panel. You need to be logged in.
You cannot change the FTP username in the Confixx Control Panel.
This is always the same as your web user webXXX (e.g. web123). You can create additional FTP users in the Confixx Control Panel, but you cannot choose the names yourself. You do this under “Settings – FTP Users”.
Note that they can only access subfolders and not the main /html folder.
In the Plesk Control Panel you can click on “FTP access” under “Websites & Domains” under the respective domain. Click on the user you want to change. Here you can change the username.