Go to the module where the Smart Rule or Smart Event has been created, and open the screen that it applies to. Example - if you have a smart event that sends an email every time an AP Vendor name is changed, go to AP Vendors. If you have a smart rule about a Project - go to the Project module, project list.
Click on Object Definition, and then on the pink bar that appears, click on Smart Rule or Smart Event (whichever you need to modify). Locate the specific rule and click on Edit. Click Next until you get to step 3 and change the email address. Click Save.