First you need to press the window +r on the keyboard and type over there Cmd.
Now type IP config in Cmd prompt and press enter. And you will see the default gateway over there so that is the IP address of the net gear d 6200.
Pull a browser and type the IP address on the long search bar and press enter .
And than it will asked for your username and the password and then it will take you to the login page and from there you can go to the administrations and update the firmware of your net gear D 62oo router.