How to Upgrade Sony Ericsson Xperia U to Ice Cream Sandwich

First of all you need to extract the firmware files that you have already downloaded. You would have noticed that the files are in a ZIP format. Extract them until you get a file with .ftf extension.
Install the flashtool on your computer.
Now browse to the directory of your computer to which the flashtool has been installed. Generally it is C:\Flashtool\firmwares. And then copy the firmware file that you have extracted in the previous step there.
Now execute the flashtool utility as an administrator so that you have full access and permissions for its operations.
If there are any Sony update services or Sony PC companion services running, you need to terminate them all. You may notice them in the notification area if they are running and exit them before you start the procedure.
In the flashtool, you will notice a thunder icon as the one shown in the image below. Click it to begin the process.

Soon you will be presented with a popup window that will ask you for the desired mode. In that window select the ‘Flash Mode’ by ticking it.
Now press OK.
Now you will see a firmware selection window in front of you. You need to browse to the directory where you had saved the firmware earlier, in case you had not saved in the flashtool firmware directory. Once you choose the desired firmware file, click OK.
Now you need to choose the correct file from the options that you see next as the one shown in the image below:

If you are using a ST25a, you need not modify any of the other options.
But if you are using the ST25i, you need to tick the ‘exclude baseband’ option as shown in the following image:

In order to begin the installation process, click the OK button that you see at the bottom of the window as shown in the following image:

If your phone is already connected to your PC, you need to disconnect it before you go further with the process.After you have disconnected the phone, switch it off.
Now, wait for almost 30 seconds and then connect your phone to the PC while it is still switched off. While you connect the phone to PC, you are supposed to hold the volume down button.
Though you will see a notification window telling you to disconnect your phone from your PC and to press and hold the back button, but you need to ignore it. So do not pay heed to this window and it will automatically disappear.
Now the flash process will commence and you can see the progress in the progress bar on the bottom of flashtool window.
During the process, do not use your phone or interrupt in between to avoid any damages or in the worst cases bricking your phone.
Once the flashing is complete, turn your phone ON.
It might take some time for the first boot and voila! You are now running ICS on your phone.

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