unit testing - How to write a JUnit test in Java, that checks one particular print that depends on a userinput in a loop

I try to write JUnit tests in IntelliJ for an application, that modifies a String. Now how can I test one particular print on the console?

Let's assume the start-string is stored in args[0] as "Hello World" and we have a method that can change a single letter in the string. The string will be printed out after the execution of the method.

public class modifyString {
    public static void main(String[] args) {

          Scanner scanner = new Scanner(System.in);

          while(true) {
              // Print the string

              System.out.println(args[0]);

              // Wait for a userinput that only contains two chars,
              // e.g. '0h' <- 'Change the index 0 to 'h''
              // (the order of the chars is unimportant)
              String input = scanner.nextLine();

              if(preconditionFullfilled(input)) {
                  executeOperation(input, args);
              }

         }
    }
}

For example: The expected output for the input '0h' with a 'Hello World' string in args[0] should be 'hello World'.

How do I write a JUnit test to test this particular result?