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I can not install 34.4.A.2.85.
Aftel downloading it. Phone reboots with the message, that I can't install it.
I would try using the software repair in the Xperia Companion app on a PC or Mac. Be sure to backup everything first if possible (use the backup on the phone to the SD card, or do so using Xperia Companion on the computer).
Follow @pseudonym suggestion
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
Trying to update using PC Companion I have the massage on PC screen:
"kResultCode_DeltaUpdateResultDidReturnFail"
If so then read the instructions carefully and pay special attention to the second post of the instructions point 2B.
Report back please if it worked for you.