if you are using Excel 2007 click on the View tab of the ribbon, and click the arrow underneath the Macros button on the right and click view View Macros. Select the Macro you want to edit, and select edit.
If you are using Excel 2003 click on Tools, Macros then edit Macros. Select the Macro you want to edit, and select edit.
At this point the macro editor opens and you can edit the macro as needed.