I have index.html file in "WTProject" Folder. I've created 404.html in "404" Folder which is in "WTProject". It simply go to 404.html from index.html.
but I don't know how to go back to WTProject Folder to access index.html. I mean what should I write in Home so that it will go in previous directory and open index.html.
You can use ../ to access previous directory in HTML. For usage check out: https://www.w3schools.com/html/html_filepaths.asp