Has to change the bulbs in my dash a while back - Assuming its the instrument panel ones - you'll need to remove the shroud surrounding the steering column and instument panel - disconnect the speedo cable at the gearbox to allow you enough slack to pull the instrument panel forward towards you.You'll be able to gain accees to the bulbs at the back.
If it's the ones on the heater controls good luck - they modified them a few years back and made the job to replace them even worse that it was - best way was to follow the instrutions in a Haynes manual - I took mine to a friendly Vauxhall mechanic and he changet them for a tenner.