You cannot change a Document Template directly. To change a Document Template you must create a Document Definition based on that Document Template, make the changes to the Document Definition, and then and then save that Document Definition as a new Document Template.
To change a document template:
1. In the New Document pane, right-click the Document Template you want to change, and then click Create Document.
2. Make the changes to the Document Definition you have created.
3. On the File menu, click Save As Template.
4. From the confirmation dialog, click Yes.
5. In the Template Name box, type the name of the Document Template you are creating.
Note that you cannot overwrite your original Document Template.
6. In the Description box, type a description of the Document Template you are creating.
7. Click OK.
After creating a new Document Template you can optionally delete the original Document Template.