Can I go from Heathrow airport to Kings cross station during 5 hours Layover?

I would not, unless you have domestic flights only. It will take some time to get out of the airport, 30 minutes is normal, it can take more than an hour if you are in a long queue for passport control.

On the way in you have to be there well before the flight, how long depends on your destination but I personally would not plan on less than 2 hours whatever your destination (which does include about 30 minutes for a missed train.)
With domestic flights you likely need less time for getting out of the airport and you may arrive with a shorter time for security. But it will still be very tight.

Travel time by train or underground is about an hour, add to that waiting time and time to visit the station and its special platform entrance is very short, you might already be in minus hours.

While it is great to see the Potter locations, it is not worth missing your flight.

(Travel times taken from comments on the now deleted other answer.)

In addition to Willeke's answer:

You can take the Heathrow Express to Paddington. This explains getting to Heathrow from King's Cross but you'll be able to apply it in reverse:

For the fastest route from Kings Cross to Heathrow simply take the underground (on either the Hammersmith and City line or the Circle line) directly to London Paddington. After your arrival, follow the signs and take the Heathrow Express from London Paddington straight to the airport.

The Express is £37 takes roughly 15 mins and you'll need to pay from Paddington to King's Cross.

Time: I always assume disembarking a plane takes 20-30mins, customs 30mins, getting around the airport (it's huge! look up the floor plan), travel by/locating the train/underground, entering security again, etc... So overestimating time, it leaves less than 1 hr to view the platform/take a picture/shop.

I'd like to note by experience that the wait for a photo at King's Cross (trolly and House scarf) can be minimum 45 mins. The shop is cute and you'll be able to pick up some great souvenirs but isn't as diverse as the studio. Edit: The Harry Potter Platform 9 3/4 website has more information: You can purchase a VIP pass to quickly bypass the long line. It's free to take your own pictures but you have to pay for a professional print. Found on the FAQ:

Do I have to pay to take my own photographs at the trolley?

You do not have to pay for your own photographs at the trolley. You can however purchase your professional photo in store should you wish to.

If you're able to go to Terminal 5, you can visit the Harry Potter shop there. Not the same as a photo entering Platform 9 3/4 but if you're strapped for time, it's not a bad alternative.