Although Irene is completely correct in suggesting the Search and Find tutorial, as a person who also creates Word files that are hundreds of pages long, let me add some additional information and simplify the process for you.
To make this process extremely easy, I put the 'Search and Replace' icon in my QuickAccess Toolbar. Here's how you can use it:
If you want to just 'find' the word/phrase, click the icon, then 'Find,' then write in the word or phrase. Be sure to select "More" if you want additional options (for example to match case, find whole word only, etc.). Then, you can either click on "Find Next" if you want to check each time one of those words/phrases appears, one at a time, or "Find In" if you want to check the current selection or the entire document. You can edit and make whatever changes you wish to the word/phrase and then continue on with your search to the next one.
Or, if you want to find the word/phrase and replace it/them with something else, select 'Find and Replace.' Again, fill in the blanks -- what you're looking for and what you want to replace it with, and again, check 'More' if you need further options. Then, by either selecting Find Next or Replace All, you can make the change(s) to the text either, one appearance at a time, or all at once universally, throughout the entire document.
And PS, I also put 'Undo' in my QuickAccess Toolbar, so that if I screw up, which is easy to do, with just one stroke I can just click Undo and undo all of the changes I've just made.