You can modify an existing Magic Launch rule after it was created.
Choose the Apple menu > System Preferences, then click on Magic Launch.
Select a file type for the rule you want to edit in the list box of managed types on the left.
Select the rule you want to edit, then click on the button with a pencil icon below the list box on the right. This will open the rule editor.
See the Rule editor topic to learn more about defining a rule.
Tip: If you only want to change the application associated with a rule, you can simply click on the appplication name on the right side of the rule list box in the preference panel. A menu will appear allowing you to select a new application.