When you are searching for a certain string in multiple files the last thing you want to do is to open each file individually and change each string one by one in each file. Notepad++ allows you to search and replace in multiple files are once.
First open Notepad++ and press ctrl + F to open the find screen, then click find in files.
In the screen that appears enter the string you want to find, the what you want to replace it with. Leave the filters as the default and then in directory enter the path where you want to implement the find and replace. When ready click Replace in files
Click ok to allow the replace to take place.
You should then see a message that indicates how many occurrences were replaced.