Thanks. I never learned how to use headings or styles and originally formatted everything manually. It then became a mission to go back and try to add styles so I just carried on doing it manually 
It's easy and it will save yourself a ton of time in the future.
It can be time-consuming to change from no styles to using styles because you'll have instances where you need to uncorrect things you did manually. You may also come across things such as italics. Styles may override any words you have previously italicized. (Which is a good argument for using styles to begin with.) When switching from not using styles to using styles, what I've done is highlighted paragraphs and watched as I scrolled and highlighted. When I came across something that was italicized or bolded (rarely), I'd stop. Then I'd switch the selected paragraphs to whatever style they needed to be set in. Then, I would move to the paragraph with italics and keep track of what words were italicized. Then, highlight that paragraph, select the style and go back and re-italize words if necessary.
I should note I use Pages not Word so Word may behave differently with regard to handling italics when switching styles.
But it is well worth the effort to learn how to use styles and use them. I used to be like, why? I didn't see the benefit. But, I learned later how much of a benefit it is and wished I had learned and used them years earlier.
Correct earlier stuff only as absolutely necessary.
The problem with that, if you make additional corrections in the future, it can become hard to keep track of what had and hasn't been corrected, which can create additional work. Better to do it right to save yourself more hassle in the future.
I know because past-me has really left of mess of stuff for present-me—then future-me—now.
