First, make a backup to your phone, as it will erase internal memory, including contacts.
Use Flashtool and search with integrated Xperifirm for previous firmware provided by your operator, or Central European version.
Download that firmware and follow the instructions. Be sure to mark wipe user data, otherwise the phone will not start.
Sony guys are kidding us when afirming that we can't downgrade to kitkat.
They fooled us with this phone. Bugs everywhere, and new others appearing with every upgrade.
I think we could ask our money back and buy something else.
