Jon --
Click File - New, click the "On my computer" link in the New Project
sidepane, click the Enterprise Templates tab, select the template you want
to edit, and then click the OK button. Edit the template as you wish.
Click File - Save As. In the Save to Project Server dialog, click the Type
pick list and select the "Template" option. In the Name field, enter the
exact same name for the template (look at the title bar at the top of the
Microsoft Project application window and you can see the name) and then
click the Save button. When prompted in the warning dialog, click the OK
button to overwrite the existing template. Hope this helps.