Actually what meintsi means is not the template but the sheet format. So you open a drawing using the sheet format that you want to change, make the changes then do a save sheet format. You don't need to save the drawing. Then open the drawing you want to update the sheet format, right click on the sheet, select properties, browse to the folder where your updated sheet format is located, make sure you have selected CUSTOM in the box above then click on "reload sheet format". If the size of the sheet format is different you mau need to chage the sheet size as well.
Andrew