I’m planning to add a command to allow an op to change the password of a player, but for the moment, you must have access to the database: you can delete the line corresponding to the user, he will just have to register again.
If you use MySQL, it should be easy for you, as you probably have PhpMyAdmin installed (or maybe you are good enough in SQL).
If you use H2, you must use a database explorer (you can find the official one here)
in both case, you can use the following SQL command:
DELETE * FROM <table> WHERE username='<user>';
Where “<table>” is the table name you selected (it is “authentic” by default) and “<user>” is the username of your player.
It may be easier for you to use the web interface directly instead of SQL commands. Make it to your preference.