You can`t update your phone with downloaded fw by SEUS.

What is the Sony Ericsson Update Service?

The Update Service allows you to download the latest software available for your phone whenever you wish ( it updates your firmware, if were some fixes for bugs released. It doesn`t update it to newer version ).
The reason for using this service is that your phone is an advanced piece of electronics, comparable with a computer, and the latest software allows it to operate in the best way possible.
Nice day