With the IconPackage applied:
1 - Start IconPackager
2 - Select Icons and Cursors in the left pane
3 - Select Files on the top of window that is now displayed
4 - Find the icon you want to change, and then use the browse features to find and assign the new image
5 - Use File > Save as.., to save the new package under a unique name
6 - Restore all Icons using the Tools menu
7 - Go to where you saved the new package and double-click to install
Note - items #6 & #7 are the conservative approach. You can also save as a new name, and if the package is displayed in the packages list, you can apply directly from there.