Java question (DL Degree - not school homework!)?

I'm doing a DL degree and we are doing a Java module. I'm completely stuck and need some pointers. Obviously I am also working full time (hence distance learning) so I don't get any lab sessions for help.

I'm pretty sure I need to use an Array in some way but I don't understand how.

This is the question

Write a Java program which takes a sentence from user input, remove all non-letters, reverse it and change case of every letter. For example: Welcome to C4DM -> mdcOTEMOCLEw

This is the code I have so far which does everything except reversing the case:

import java.util.Scanner;

public class StringTest2{ public static void main(String[] args){

Scanner myScanner = new Scanner(System.in);

System.out.print("Please enter a string: ");

String s = myScanner.nextLine();

String nInput = s.replaceAll("\\W", ""); //remove non word characters

String oInput = nInput.replaceAll("\\d", ""); //remove digit characters

String rInput = new StringBuffer(oInput).reverse().toString();

System.out.println("Result: " + rInput);

}

}