Simply open it. If it truly is an INDT file, you'll be presented with a new Untitled document, with the contents of the template.
If it is not, you can simply Save As.
According to how I read what you wrote, you received an InDesign file, .indd, and saved that as an InDesign Template, .indt.
The original file may have had Styles and Swatches. Saving as an indt would not have removed them, if they were in the original document, they would be preserved.
The only idea worth note, is that upon opening the indt, the document will always open as Untitled, requiring a Save As.
If you have to use these Styles or Swatches in other documents, you can choose to, via,
From the Swatches Panel flyout (downward arrow) select Load Swatches, navigate to the indt file and select, the swatches will be added to the current document. The same technique can be used for C and P Styles
Sorry if not clear. Styles and swatches I need are in my pre-existing document file, 110 pages containing the book content. I am now creating the cover in a separate file, and would like swatches and para styles. etc., to migrate from existing book to the new cover document.