From the LibreOffice Writer 5.4 Guide, Chapter 10, Working with Templates, pp. 227–228:
Associating a document with a different template
At times you might want to associate a document with a different template, or perhaps you’re working with a document that did not start from a template.
One of the major advantages of using templates is the ease of updating styles in more than one document, as described in Chapter 9, Working with Styles. If you update styles in a document by loading a new set of styles from a different template (as described in Chapter 9), the document has no association with the template from which the styles were loaded—so you cannot use this method. What you need to do is associate the document with the different template.
For best results, the names of styles should be the same in the existing document and the new template. If they are not, you will need to use Edit > Find & Replace to replace old styles with new ones. See Chapter 2, Working with Text: Basics, for more about replacing styles.
-
Use File > New > Templates. In the Templates dialog, double-click the template you want to use. A new document opens, containing any text or graphics that were in the template.
-
Delete any unwanted text or graphics from this new document.
-
Open the document you want to change. (It opens in a new window.)
-
Use Edit > Select All, or press Ctrl+A, to select everything in the document.
-
Click in the blank document created in step 1. Use Edit > Paste, or press Ctrl+V, to paste the contents from the old document into the new one.
-
Update the table of contents, if there is one. Close the old file without saving. Use File > Save As to save this new file with the name of the file from which content was taken. When asked, confirm you wish to overwrite the old file. Or, you may prefer to save the new file under a new name and preserve the old file under its original name.
Caution
Any changes recorded (tracked) in the document will be lost during this process. The resulting document will contain only the changed text.