Roll back firmware update I've installed frimware update on my phone , but it seems display drivers have some bugs.

How can I rollback display drivers or at least the whole update?

My phone is: "I-mobile IQ 5.5".
There is no easy way to roll back an official firmware update.  Generally when carrier or smartphone manufacturer releases an update, they do not provide a way to downgrade.  You can try to find the original firmware on the Internet, or try to install a custom firmware of the previous version, if one is available for your device.  Both these workarounds will most likely require you to root the phone and possibly unlock its bootloader, and will most likely void your warranty.

Regarding your particular issue with display, it's possible that a simple factory reset will resolve this.  If that fails, you should take the phone to a service center and have their technicians examine it for defects.  If the phone is still under warranty, you may want to send it for repairs.