Courtesy of Digital Spy Forum
(1) Put box into Standby;
(2) Remove the viewing card and put it back in the same slot UPSIDE DOWN (remember which way up/round it was for later (see (9) below);
(3) Press and hold the Back-up button* on the Digibox (not the remote control);
(4) Whilst holding that Back-up Button, switch off the power to the box at the mains - either with the switch on the socket or by removing the 3-pin plug from the wall socket;
(5) Count to ten slowly;
(6) Keep pressing that Back-up button and put the power back on to the box;
(7) Keep pressing that Back-up button and, when all four lights appear on the Digibox, release it (if your Digibox is connected by scart, there will also be a message on the screen stating an update is taking place and that it can take 10 minutes [but it can take longer]);
(8) When the box goes back to a single red light, leave it at least a full minute before taking it out of Standby using the Sky button on your remote control;
(9) Remove the upside down viewing card and replace it the right way up (make sure you put it back as it was when you first removed it);
(10) The box will switch to channel 998 - the Sky demo channel (it may say “Searching for listings” - don't worry - wait);
(11) When that clears, do a Services – 4 – 5 check to see the Operating System Version and/or EPG Software Version numbers have updated (the Model Number may also have changed);
*If you power off first and then press Back-up, it does not clear all the old software - so follow the above exactly for the most reliable results.