On a ex ntl Samsung box:
1. Unplug the mains.
2. Hold down <, CH ^, >
3. Plug the mains back in keeping the buttons held down.
4. After about 10-20 seconds you can let go of the buttons.
It should say the STB is updating and the display on the front of the cable box will be counting. After a while the box should be updated.
These instructions should work for ex tw Samsung box too.
Hope this helps.