Ok, first if you havent already, and I can almost guess you haven't you need to download PC Companion and Sony Ericsson Update Service (SEUS) to your computer.
Once you do that, turn your phone off, and run SEUS. Follow the steps and select your phone when given the option. After you select your phone, it will tell you to plug your phone into your computer in flash mode, basically just hold the "back" button on your phone until SEUS tells you to release it. If you have a new upgrade available then feel free to do so,..if you already have the latest software then in the bottom right corner you will see an option that says "install" click on it and proceed. It will reflash your phone for you and should be ready to go once done.