1- Download Lenovo S90 u Flash file
2- Download QPST tool.
3- Install Qualcomm driver.
4- Run QPST tool.
5- Add new port (Check device manager)
6- Click Load XML and browse for rawprogram_unsparse.xml in Stock ROM file.
7- Click Load Patch and browse for patch0.xml in Stock ROM file folder.
8- Select the firmware by clicking in Browser of Search Path.
9- Select Download and wait for the flashing to be finish.