How to call a swap function from from2 to form2

If it's an instance method, then you need a reference to the form that it's on in order to call it. There are many ways to get such a reference but here's one way of doing it.

Assume that Swap is a public or internal method on Form2:

Form2 f2 = (Form2)Application.OpenForms["Form2"];
f2.Swap(ref a, ref b);