Editing a message catalog definition in PeopleSoft is pretty straight forward, simply browse to the following menu path:
Enter your message set number into the prompt. Note that if you do not know your message set number you can use the following to find it:
WHERE MESSAGE_SET_NBR = '12345'
AND MESSAGE_NBR = '12345';
WHERE MESSAGE_TEXT LIKE '%what your message is%';
Once you have your message simply edit the text and description appropriately then save!