Update happens automatically. Howver if u wanna update manually follow dis:
When ur stb is on, close it from the mains without switching off from remote.
After some time switch on stb from mains and when 'please wait initializing' comes on screen, press back button twice and then press 2465. Software will start downloading.
note- only latest software available over the air released by tata sky will b downloaded and do not try this many times as it will damage software downloader. So keep caution. And this works in SD boxes donno abt HD STB.