In WordPress Dashboard go to Appearance submenu and click on Customize.
After clicking on customize the theme Customizer page will appear .
Here You have to follow this Path : Go to Customize -> Page Templates -> 404 Page
Now scroll down till Title and here you can Add Section Title , Subtitle , Description & Button Label in 404 Section in 404 Page.
NOTE : Please First enable the Hide/Show Section to display 404 Page Section
Then click on Publish.
Please follow this Screenshot for Reference :
