How do I change the variable in a concrete decorator during run-time in java

Here's the situation.

The Items abstract class with a boolean flag:

public abstract Items {
    public boolean flag;

    public abstract void display();
}

A concrete implementation:

public ConcreteItem extends Items {

    @Override
    public void display() {
        // do things...
    }
}

The decorator:

public class ItemDecorator implements Items {

    public Items item;

    public ItemDecorator(Items item){
        this.item = item;
    }

    @Override
    public void display() {
        this.item.display();
    }

}

and the concrete decorator:

public class ConcreteDecorator extends ItemDecorator {

    public ConcreteDecorator(Items item) {
        super(item);
    }

    @Override
    public void display() {
        super.display();
        System.out.print("Adding decorators to Item...");
    }
}

So when I create an Items object decorated by a ConcreteDecorator, how could I to change the variable flag in ConcreteDecorator in run-time?