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How are you trying to update/reset the phone? If you want to reset it, you could perform a software repair on your phone via PC Companion and the latest update will be installed:
http://www.sonymobile.com/global-en/tools/pc-companion/
In PC Companion you press start on Support Zone > Start on Update the phone/tablet software > Repair phone/tablet (the blue clickable link in the message) > Follow these steps without having your device connected to the PC.
Follow the instruction that appears in the program and do not connect your device until this guide tells you how to connect it.
