Listbox in windows forms application

Hey,
I have a listbox in my Windows Forms Application. I add items to it using

Code:

---

listbox->Items->Add(...)

---

Once I have added an item, can I change it (to be specific, change a member of the object that I've added) without having to remove and add the object again? I have a custom ToString() method that depends on the values of the members of the object, so I want to have it so that modifying the object changes the text that appears in the listbox. Do I do Invalidate() or Update() or Refresh() or something like that? I've tried those three but they don't seem to work...
If this is not clear, please say so and I will do my best to explain my question better.
I'm using C++ with Managed Extensions in MSVC 2003, if that matters.
Cheers,
Philipp