How should I change a class value from another

I am learning to program in Java, and I want to know if it is a good concept of OO programming to change a value to a composed class from another composed class. Like this:

public class X{
    public void x(Y y){
        y.setY(0);
    }
}

Or should I appeal to the Main class? Like this:

public class X{
    public void x(Main m){
        m.modifyY(0);
    }
}

public class Main{
    private Y y;
    private X x;

    public void modifyY(Main m){
        y.modifyY(0);
    }
}

PS: I am learning UML too, so I'm sorry if I'm making it wrong.

You can change like this,

public class X{
    public X(Y y){
        y.setY(0);
    }
}

public class Y{
   public int val=0;
   public void setY(int p_val){
       val=p_val;
   }
}

public class Main{
    public void modifyY(){
         new X(new y());
    }
}