iphone - How to insert only one username and password in SQLite database?

I have an application where I have two textfields and a button. In the first textfield I am entering the username of the user and in the second textfield I am entering the password of the user. When the user enters the username and password and click on create user button a backend api is called which will register the username and password of the user and at the time I want to enter the username and password of the user in local SQLite databse in encrypted format.

When a user is registered the username of that particular user should get displayed in the first textfield and it should not be editable and password field should be editable. Only one user is allowed to be registered for the entire application.