Xperia Pro/Neo/V/L :: Xperia Pro Mk16i Will Not Upgrade To ICS?

I decided to upgrade my Xperia Pro (SI 1252-5232, it´s in the list) to ICS. the Sony PC Companion sees the phone and writes an update to version 4.1.B.0.587 is possible, so i click on agree, it downloads the SW to the PC, and than it TURNS OFF THE PHONE and shows a step-by-step guide how to update (1. disconnect USB cable from phone and PC, 2. battery must be +50%, 3. turn off phone and wait 5 sec.->>???WTF? how can i turn it off when "it" turned it off itself? 4. connect USB cable to PC, 5. hold "back" key and connect phone to USB). I tried it several times, with MTP and MSC mode, disconnected it, turned off, waited 5 secs, pressed "back" and connected" maybe 5 times all the possible ways, but it simply does nothing when I press the back button and connect it, only the LED is green.

I´am doing wrong? How long should I press the back button, which phone-settings are required, where could be the error?