You can modify Costing templates any time there are changes in cost structures. For example, if the price of steel increases, you can change the cost in the template.
If you want to make a temporary change instead of changing template data, you can override values in the CostingManager or Machining Costing Task Pane.
Default templates are read-only. You cannot overwrite a default template, but you can save the template as a different name.