Changing firmware in the phone will erase all of your data.
All described operations you are doing at your own risk.
If want saves your data do a backup before start.
Battery must be at least 60% if not you will kill phone completely.
1. First of all, we need to find out exact phone model that we've got.
also, you can find it in Xperia GX phone menu as shown below
7. Download firmware by pressing firmware version button.
10. Select Flashmode and confirm with OK button.
11. Select SONY Xperia GX phone model and model number of your phone, firmware customization and firmware version the one you've downloaded in step No. 7.
14. Connect turned off Xperia GX phone to USB cable with pressed volume down button.
15. Release volume down button when you see "Device connected in flash mode" information in FlashTool windows log.The flash process starts, now just wait until process end.
16. After all disconnect Xperia GX phone from the USB cable. Turn it on with the power button and leave it on a flat surface until phone turns on completely
(this is very important otherwise the phone will not complete the first start and you will see an error message on the phone screen).
17. Congratulations your flash process ended successfully. Now you can enjoy your SONY Xperia GX with new firmware.