Step - 1 Login to your cPanel.
Step - 2 Under the Files section click on File Manager.
Step - 3 Navigate to the document root folder for the site.
Step - 4 Click on Settings.
Step - 5 Ensure Show Hidden Files is selected.
Step - 6 Then click Save.
Step - 7 Right click on the .htaccess file and select Edit.
Step - 8 If the .htaccess file didn't exist in the previous step, click on New File at the top left, name the file .htaccess, and set the directory for creation to /public_html/ or the document root of your site.
Step - 9 You might have a text editor encoding dialog box pop-up, go ahead and just click Edit.
Step -10 Now that you are ready to edit the .htaccess file, the simplest way to set a 404 error page is by directly setting a 404 error message in the .htaccess file itself:
ErrorDocument 404 "<H1>Page not found</H1>"
If you already have a static HTML page that you'd like to use for 404 errors you can also direcly point to that file with the .htaccess file as well:
ErrorDocument 404 /404.html
Step -11 After you've made your edits, click on Save Changes, and you should see a green Success pop-up
Step -12 Now if you try to pull up a page that won't exist on the server such as /ThisPageWon'tExist you should see the custom 404 error message that we've configured. The first image below is the default 404 error page when a custom one is not set, and on the second one is our custom one that we've just created:
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