The Word 2016 Replace command's dialog box sports the handy Replace All button because it's tedious to use the Replace button when replacing text. Sure, sometimes it's handy, especially when you don't want to replace everything. But if you've changed your novel's main character name from Xlaborded to Zlaborded, you can probably and safely replace every instance all at once.
The Replace All button directs the Replace command to find all instances of the Find What text and — without question or pause — replace it with the Replace With text.
Be doubly certain that you made the proper settings in the Find and Replace dialog box before you click that Replace All button! You can still undo any mistakes, but for a large document, a lot of text can be found and replaced in a manner most merciless.