A family of Microsoft word processing software products for creating web, email, and print documents.
I agree with John that the best method is to simply create a new document from the template.
Unfortunately, that does not deal well with transferring a set of styles to an already typed document.
One method that can sometimes work is to create such a new document, create a paragraph in each of the styles so they are firmly in use, and then copy the content of the existing document into the new one, merging formatting. Then delete the dummy paragraphs.