4. how to unlock your bootloader
a) download and install latest Flashtool from here (mirrors Google Drive, Ulozto or Mega)
b) start Flashtool and click on BLU button on the top
c) turn off your phone and when Flashtool shows pop up window, connect phone to PC with holding volume down button
d) click on get unlock key button, it will switch you to Sony Mobile website, here select your phone and hit continue
e) fill your email adress and accept terms and conditions, than click submit
f) go to your mailbox and click on Click here to proceeed link in mail from Sony Mobile
g) copy IMEI from Flashtool to Sony Mobile site and once more accept term and contidion, than click submit
h) copy unlock key to Flashtool window and than click unlock
i) connect phone with holding volume up button when Flashtool shows pop up window
j) wait to unlock finish