1. Log in to cPanel and go to the main menu, where you will see a link to the 'Error Pages' section and follow that link.

2. Click on the button of the required error page, such as the 403 button. After following that link, you will be able to easily upload your custom '404 Error' page using a built-in web based HTML editor. You can use the buttons at the top of the page to insert variables into the displayed code.

3. Finally click on 'Save' the button.