You will probably have to remove the sight.  Two screws.  Soak them in penetrating oil if they don't move.  Not familiar with the pin that Dick is referring to, but it should be easy to detect once the sight is removed.

You many simply find that the sight is stuck to the base.  Detatch the slide from the base and lightly clean the mating surfaces with fine steel wool and oil.  Once it is cleaned and lubed, turning the windage lever will remove tension from the sight and you should be able to move it from side-to-side.