When you type text into the Quick Find field in the story map's toolbar, passages whose text matches what you've entered will turn yellow. Use the window icon on the right side of the search field to bring up a fuller-featured dialog box.
Typing text into the Search For field will instantly show a list of matching passages. To see the exact match, use the plus sign icon beside a passage name. You can also quickly show and hide match detail with the icons beside the Replace All button.
Below the search field are a few options:
To replace the text you're searching for with something else, enter it into the Replace With field. You can use the Replace All button to do all replacements at once, or use the Replace in Passage link that appears when pointing at a passage with the mouse, or tapping it on a touch-based device, to perform the replacement in a single passage.
When you're done, you can close the Find and Replace dialog with the close button in its upper-right corner or by pressing Escape.