From CRXI help:
Once you've created a custom function, you can view or modify it at any time using the Custom Function Editor.
When you view a repository custom function, the Custom Function Editor and the Custom Function Properties dialog box appear in read-only mode. (You cannot modify a repository custom function without first adding it to your report.)
To modify a custom function
In the Formula Workshop, expand the Report Custom Functions node and select the custom function you want to modify.
Note: If the custom function appears in read-only mode, you may have it connected to the repository. Check the Workshop Tree for the "connected to repository" icon. Disconnect the custom function from the repository to make it editable.
You can modify the text of the custom function by using the Custom Function Editor. You can modify the custom function's properties by using the Custom Function Properties dialog box. Switch between the two interfaces by clicking the Use the "Toggle properties display" button. For more information on the syntax of a custom function and rules associated with custom functions, see Custom function syntax.
Make the necessary changes to your custom function.
Check and save your changes.
Note: If the custom function you're modifying has been added to the BusinessObjects Enterprise Repository, be sure to update the repository copy when you're done.