Instructions
Press the "S" key on your keyboard at any time during a game. This will expand a list of spells on your screen.
Left-click on the icon for the spell you want to change. Click on an enemy within the world of the game to cast that particular spell.
Press the "ESC" key on your keyboard one time to close out of the list of spells on screen. Press the "ESC" key again to bring up the "Diablo 2" menu. Click "Save and Exit."
Click "Play Game" from the main menu. Select the icon for your character to re-enter the game. Press the "S" key on your keyboard. The order of the spells on screen will be changed. The spell you most recently cast will be at the front of the list.