Once a document/letter template has been created for mail merge purposes, you can edit the templates via the Mail Merge Wizard.
You may want to edit a template if there is a spelling mistake or you need to update a year reference contained within the body of text or to add additional information to your document template. You can also add or remove mail merge fields in your document templates.
If you want to edit letter templates, you should do this from the Mail Merge Wizard in AE/AO and not directly in Microsoft Word.