Manage Not found post

If you click any post title in your blog then it will redirect you in a page where you can see this particular post in that case your browser address bar show you "/?p=50" suppose your post id 50. But if you directly put the id of the post in your address bar and if it's not available then it can show you an error
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Not Found

Apologies, but the page you requested could not be found. Perhaps searching will help
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Now if you want to change the design of this page go in your theme folder and edit "404.php" file