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There are many official ways how to update firmware on your Sony (Ericsson) XPERIA smartphone. You can use PC Companion or Sony Update Service on Windows computers or you can use Sony Bridge for Mac on Apple computers. But sometimes it is better to use unofficial way - you can get latest firmware as it's released for your country etc. Androxyde Flashtool is the best PC utility, which allows you to install new firmware, decrypt firmware files from Sony or relock bootloader on your phone. And it's available for Windows, Linux and Mac.
Androxyde Flashtool is very powerfull software, which allows you to make many advanced things with your phone