The Calendar Printing Assistant uses templates with the extension .CATX. The templates are stored in the Office template folder, which is typically the C:\Program Files\Microsoft Office\Templates folder.
These files are essentially renamed compressed files that can be extracted and edited. In order to do this, copy the .CATX file you are interested in editing to a different location. Then rename the file to have a .ZIP extension. After renaming it to have a .ZIP extension, you can extract the files to be edited. You will see something similar to the following:
These files will all be Read Only, so change the props so that they are no longer Read Only for editing.
I like to make a copy of the original CATX file so I can scrap and start over again if needed.
The really important files to edit are Content.xml - located in the \Cal directory, and Metadata.xml - located in the \Cal\Metadata directory.
Content.xml is the main file for the layout of the page and controls how appointments and tasks are seen. Metadata.xml controls page size, the name of the template as seen in the Calendar Printing Assistant, and other general template layout info.
When you are done editing, you will want to zip up the _rels and Cal folder along with the "[Content_Types].xml" file back into a new ZIP file. Then rename that file to have a .CATX extension and place it in the C:\Program Files\Microsoft Office\Templates folder. Then it can be used in the Calendar Printing Assistant.
If you replace the same file/template name in CPAO - you do not have to restart the program - just switch templates in the app and then switch back to the one you were editing and the updated template will show (or maybe not if your editing didn't work!)