If you are using another program to create your Template HTML or design the visual look and feel of your document you might have occasion to completely replace the Template HTML. To do that you can use the Import HTML from another source option in the Utilities menu of the HTML Source Code panel.
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In the Import HTML window use the 'File' menu to open the appropriate file you wish to import.
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If you're satisfied with the "look" of the results click the 'Ok' button.