Your question is a bit hard to understand.

But that is the whole purpose of styles.

You apply a style (Heading 1) to your text and then if you ever change the style (Heading 1) (eg from Blue text to green) then every piece of text that you have applied that Style to changes.

But yes of course you will have to manually apply the style to the text that you want on your 100 pages.

(If you had known about styles earlier you would/could have applied the Heading 1 style to the text as you created it!)