The 1st thing to do would be to measure your front sight height with a precision pair of calipers. If you go to Dawson Precision website (link http://www.dawsonprecision.com/CategoryProductList.jsp?cat=SIGHTS+FRONT
) it will show/tell you exactly how to do this.
You would then need to order your replacement (I recommend from Dawson - they offer brass bead & fiber optic replacements) which matches the height of the one that is on there now.
You would then need to remove the front sight by first driving out or removing the stake pin and then pushing the front sight out of the dovetail on your slide. You will most likely need a sight pusher tool (you can order these from Midway or Brownells). Dawson also has detailed youtube videos to show the entire process. It's not that hard to do if you have the right tools and are patient.