I have a Nokia Lumia 520, which has nearly the same hardware configuration as the Dell Venue Pro has, but the thing is that the Venue Pro has Windows Phone 7.5 as its built-in OS. Can I upgrade it Windows Phone 8.1?
No, you cannot upgrade the Dell Venue Pro to Windows Phone 8.
No Windows Phone 7 device is able to run Windows Phone 8 due to hardware requirements.
No, You cant due to hardware limitations. You can however upgrade to WP 7.8 which supports smaller tiles.