Use this tutorial XS++ 3.1 Guide-Flashing, Customizing, Recovering - Top Sony Forums and this firmware files:
MAIN- W850_R1KG001_MAIN_GENERIC_CY_RED52
FS- W850_R1KG001_FS_EUROPE_2_RED52 and this
CUSTPACK- HB1-06_Gen_CDA102568_11_R5A_GENERIC_EUROPE_2 if your phone isn't branded (this is the newest firmware )