First install the USB flash driver: SETool2Lite_folder\drivers\usb_flash_driver\gg-setup.exe
Start SETool2 Lite, select phone type(W550/W600). First backup your GDFS(this is important): click Read GDFS.
Now remove the battery, wait a few seconds and insert it back. Press and hold the C button on phone, plug the cable in phone and follow the commands in SETool2Lite.

How to flash it?
Click Add button and select the *.mbn(MAIN firmware) and repeat it with *.fbn(FS) too. In MISC files: select the customization pack(zip archive), and click the FLASH button.