I don't think you can change the attack button that easily.
The reason for that is because it is considered a skill by the game (I think), and would have to be edited in the same manner as any other skill (such as Holy Light or Animate Dead).
The problem with that is that attack skill is hidden in the editor (such as some other skills I cannont remember right now).
The only ways I know of editing it are the following:
1. Widgetize your map and edit the attack ability in the txt/slk files.
2. Widgetize your map, find the "hide in editor" part of the attack ability and set it to false. Then you can somehow de-widgetize the map (I don't know how) and edit it back in the editor.
Unless someone else knows another way to do this, you might as well give those things a try.