I have a key guard(used to protect the laptop's keyboard). When it was new, it looked nice and clean. But now it had started to turn opaque, not completely but to the extent which could make it look dirty. How should I clean it and make it somewhat better?
That appears to be some type of semi-transparent vinyl material or something closely related. Whether it is literally vinyl or not, the chemical composition that makes the vinyl clear and flexible is what also renders the material less resistant to discoloration and UV damage. The gradual loss of transparency over time is due to bodily oils, light, and particularly sunlight.
That type of damage is not reversible from a practical standpoint. Preventative measures include limiting the installation time to minimize light exposure (not practical), and to wipe the surface with a non-abrasive cloth after use using a gentle, highly diluted detergent like dish soap mixed with lots of water.
Either measure isn't terribly practical for a low-budget item that is designed to be convenient for day-in and day-out use. An occasional, light cleaning will help, but in practical terms, I would consider this a disposable item you use until it no longer suits your needs… and then replace it.