Here’s how to download and Flash the Sony Xperia U ST25i Stock ROM Flash (FTF firmware) using the Xperia Flash Engine on a Windows PC.
Requirements: Your Xperia ™ device must have at least 30 to 40% of your phone’s battery to process the flash.
Flash Sony Xperia U ST25i Stock Rome uses FlashTool
Step 1: Before we start the flashing procedure Download Sony Xperia U ST25i Drivers from the link and if you have already got your drivers installed please skip the step.
Step 2: Download and unzip the Sony Xperia U ST25i Stock ROM on your computer. After extraction of the zip file, you can view the same file (with the .ftf extension):
Step 3: Download and install the Sony Xperia Flashtool software on your computer (if you already have the Flash tool, you do not need to download it again).
Step 4: After installing the Sony Xperia Flash tool, open it from C: Drive>Flashtool.
Step 5: When you boot Sony XPP FlashTool the following screen looks like this:
Step 6: Now click the Flash button.
Step 7: Now you can see the Boot mode selection menu. Select Flashboot and click OK.
Step 8: Load the downloaded Xperia software (.ftf) into the flash utility to do that click the Load button
Step 9: Now select the firmware loaded.
Step 10: Now press the flash button to start the process.
Step 11: Turn off your Xperia Device now.
Step 12: Press n Hold the Volume down Button and use the USB cable to connect the device to the computer.
Step 13: After successfully connecting to your computer, you will be able to see the new message in the Xperia Flashtool.
Step 14: Flashing of Firmware will then begin and end in 5-10 minutes.
Step 15: As soon as the process ends, Disconnect the Device from the computer and Turn it On.
NOTE:
Loss of data: Flashing of firmware is always a process in which you can lose data. So back up your personal data before backing up your firmware.
Sony Xperia: If you are looking for basic software for your device, visit the Sony Stock software page for all devices.
Driver Sony Xperia ™: if you are looking for an original USB driver for your device, download the Sony USB driver for all your devices.
[*] Credit: Xperia Flash tool, created by Androxyde. So, come to them to create a full loan for free applications.