If you like the stock TW Launcher you can change the email icon by going into your app drawer and tap your menu button thats at the bottom of your phone. Then click edit, you'll see your dock at the bottom zoom out and move to the middle. Now you can long press on the mail icon once you feel the vibrate you can move it back to the app section. Then you can move another app you want down to the dock.
If you don't like the TW Launcher you should try Go Launcher EX , its free in the android market, and its very customizable.
Hope that helps