www.belightsoft.com/disccover | Start of Help > Document Layout > Custom Templates for Papers and Trays |
To modify a pre-defined template, select it in the list and click the Customize button.
Then follow the steps described in Creating a Custom Template.
Note that if your goal is changing the label dimensions (not changing its location on paper, or adding/ removing labels), you can set new size on the Design Element tab in the Inspector.
By customizing a pre-defined template, you cannot overwrite it, but save its modified copy as a custom template.