Hello Chenfung
To reset your Asus laptop to its factory defaults, you must follow the instructions below:
WARNING!! - By restoring your laptop to factory settings, you will LOSE ALL YOUR DATA. Make sure you have backed up all your important information before proceeding.
> Turn off the laptop.
> Power it on again.
> Press the F9 key as soon as the Asus logo appears. (This will give you access to the hidden partition).
> On the Windows Boot Manager screen, choose your preferred language and click Next.
> Follow the on-screen instructions from there to restore your laptop to factory defaults.
Hope this helps. Good Luck!!