Alas, the information presented in the books about editing a template may not work. I don't know why, though my guess is that you cannot edit a template if there are documents open using that template. But anyway, the steps below will work in either Word 2002 (also known as Word XP) and Word 2003 to help you edit a template file already on disk.

Choose FileNew

This displays the New Document task pane on the right edge of the window, crowding out your document and annoying me to no end.

In Word 2003, click the link that says "On my computer..."
In Word 2002, click the link that says "General Templates."

Either way, this displays the Templates dialog box.

Right-click the icon of the template file you want to modify or edit.
Choose Open from the pop-up shortcut menu.

By choosing Open instead of New, you're opening the Template document (DOT file) itself for editing, as opposed to creating a new Word document (DOC file) based on that template.

Modify the template.
Save it back to disk when you're done.

The changes affect all new documents you create with that template.