Java singleton pattern - updates to instantiated variable

Looking to figure something out about the singleton pattern.

If I implement a singleton pattern like the below, what could I do so that other classes could update and see updates to fields someString and someInt?

From what I read about the Singleton pattern, immutability was not one of the prerequisites. So technically could I have setter methods for the fields and change them and have these changes visible to other classes? For example if I have another two classes implementing Runnable and printing Foo's fields every few seconds. I tried this and what happened was that each class sees its own updates only and none of the other classes'.

//generic Singleton implementation
public class Foo {

private static Foo instance;
private String someString;
private int someInt;

private Foo(){
    someString = "a";
    someNum = "1";
}

public static Foo getInstance(){
    if(instance == null){
        instance = new Foo();
    }
    return instance;
}