Macropod (Paul) is trying to tell you that this will not be easy.



First: work with a copy of your subject document. Some things may be difficult to undo.

Second: attach a template containing your styles to the document. This can be done using the Developer Tab's Document Template button. Check the button to update styles from the template. Then go back to the Document Template button and uncheck that checkbox.

Third: examine at your document. What, if anything has changed already, with just this step?

There are three kinds of styles, paragraph, character, and linked. Find the text you want to apply the style to. For instance, if I wanted to put bold text in a style, it would be the character style of "Strong." You can do a replace changing text with the font formatting of "Bold" and applying the Style "Strong."

I generally simply start with a plain text document (no formatting) in a template that has the styles I want. If then apply my Body Text style to the entire document and go through and apply other styles as appropriate. That gives me the best result but it can be a lot of work.

Another approach is to open (a copy of) your document and then apply AutoFormat to it. This will not apply styles, other than headings, though.

I would just add that nooru identified in another post that he is used to InDesign.
nooru, you need to recognize that ID is a very different application from Word. Word is a fancy word-processor, and although it has many strengths and automation features, it is be no means an graphics-suite-integrated page-layout application with the enormous power of ID. There are indeed many scripts that have been written, and can be written, for Word, many of them available commercially from third-party developers. In my experience, however, the kind of scripts available for styling in InDesign, such as nested style scripts and their offshoots, are not part of Word's overall development. The applications are designed to do different things.
I leave the expertise on such matters to people such as Macropod, whose knowledge of Word is deep and authoritative. I wrote this post in hopes that you can avoid losing time attempting to do something with the wrong tool. I have seen automated tabletop sewing machines that can embroider intricate patterns. Trying to do the same with a machine not designed for this is, at the posters have noted, very difficult.

Cordially,
Ulodesk