Once you have the software, you'll have to load it on a pen drive (its usually a zip file). Turn off STB from main power, insert pen drive and hold the power button and turn on the STB. After 10 sec. flashing will begin. You can see the percentage on screen.
It is the only manual firmware update process i am aware of on d2h. I have did it a few times on 2012-13.
Software files are available only through d2h officials and they won't provide it to normal users usually. You can rather arrange a service visit or contact near by office to manually update through them. Its chargeable if you don't have any issues with current firmware, i guess.