what you have to do is the following (in this order!):
Go to the i-doit admin center and navigate to tenants. Write down the username for each tenant in the Mysql section. Change the password for each MySQL user in the admin center GUI.
Open a mysql client and change the MySQL password for the users that you just identified (matching the passwords you entered in the admin center). Don't forget to commit a 'flush privileges;' command.
Try to login to i-doit. If you can still login, go on and change the system user password.
Open the file /[idoit_webroot]/src/config.inc.php and look at the
$g_db_system = array(
"user" => "xxx",
"pass" => "yyy
part. Change the password to a new password in the file and save it.
Open a mysql client and change the MySQL password for the user mentioned in the config.inc.php file. Don't forget to commit a 'flush privileges;' command.
Login to i-doit
Additional expert hint: The tenant database passwords are set in the 'idoit_system' database in table 'isys_mandator'.