I will tell you how to do it: run Sony Ericsson Update Service, choose your language, click Start, Click Accept, choose the style of your phone and click continue, choose your phone and click continue, follow the steps, then your phone would be detected.

Up you will see:
Latest software is already installed
Then instead of your firmware there would be written:
W910 (Origin for me CENTRAL-EUROPE) CDF1201-0116(numbers may differ)
Under It there is written:
If you are experiecing problems with your
phone, a software re-installation is recommended

All user data will be overwritten
Then click Continue Anyway