In the Administrator tab of the Project Security, you can change the default credentials and a user role for the Administrator user:
2 Administrator Properties
In the Administrator tab the following properties are available:
2.1 User Name
The user name is used to sign into the application as the Administrator.
Since this is general knowledge, it is safer to change this to a custom user name.
The password is used to sign into the application as the Administrator. Click Show password to see the password.
This password is only used when Mendix is running locally. You can change the password for your other environments in the Developer Portal.
Since this is general knowledge, it is safer to change this to a custom password.
2.3 User Role
The user role assigned to the Administrator. For more information, see User Roles.
The administrator is always created and has the System.Administrator role by default. The System.Administrator role allows users of your application to be managed. For Free Apps, the user that created the application automatically has this role by default as well so you can use it to manage your users in that environment. This role may be helpful in case you have exceeded your user license restriction in which case you can use any user that has this System.Administrator role to sign in to manage your users.