OK.. So first of all backup you ta (Ta controls your bravia engine) here is thread http://forum.xda-developers.com/show....php?t=2292598
unlock bootloader with fashtool (fashtool is easiest way) To relock just restore Ta (remember not to restore it on custom rom) that all.... And if you don't know.. you don't need to unlock bootloader to flash stock rom...
Sent from my ST25i using xda app-developers app
IF I HELPED YOU PRESS THANKS BUTTON
[DEVICE] SONY XPERIA U
[Model No.] ST25i
[Status] Rooted, BL Unlocked
[Android version] 4.0.4 (ICS)