Editing an existing canned message

Select the "Canned Messages".
In the Messages section, hover over the name of the Canned Message you would like to edit, and then click the icon to the left of the message name, and click on the "Edit" button from the drop-down menu that appears. You can also click the name of the Canned Message you would like to edit, and click Edit on the top right of the Canned Message details page.

From here you can change the message, which department it is associated with, or if it is for all departments, and what Canned Message Category it belongs to.
You can even add a new Canned Message Category directly from here, by clicking Add new category. Then enter the name of your desired category, and hit Enter to save all your changes.
Click "Update".