On the off chance that you've not replaced an icon before, its fairly simple.  First download the png and open it in preview.  Navigate to the application and get info on it (command-i). Now select the entire png image (command-c) and click on the icon inside the "info" window of the application.  It should get a blue halo around it.  Now paste (command-v).  If maya is on your dock, remember to take it off and then replace it in order for your updated icon to refresh and show up.