Today, while reading the manual of WebCalendar, I found out that I can recover the the default admin user of WebCalendar database by excuting sql
INSERT INTO webcal_user ( cal_login, cal_passwd, cal_lastname,
cal_firstname, cal_is_admin ) VALUES
( 'admin', '21232f297a57a5a743894a0e4a801fc3', 'Administrator',
'Default', 'Y' );
the password would be admin too, then I can login in to set up another admin user and remove the default admin.
INSERT INTO webcal_user ( cal_login, cal_passwd, cal_lastname,
cal_firstname, cal_is_admin ) VALUES
( 'admin', '21232f297a57a5a743894a0e4a801fc3', 'Administrator',
'Default', 'Y' );
the password would be admin too, then I can login in to set up another admin user and remove the default admin.