What to change/write in routes.rb .. ?

Dear Hemant,

First thing, please do read routes guide, and preferably a rails book. it will help you understand things better. But as far your question goes, your are seeing error because your are running your app in development mode. Try running it in production mode, and you will see a 404 page (present here: public/404.html) Customize it as you want.

Lastly, please be patient when you are expecting others to solve your problems, and yes, Google always help. So use it.