Users are not encouraged to change the bulb in microwaves for safety reasons. Radiation and very high voltages lurk inside even when unplugged!
The general case is that the outer cover needs to be removed to gain access to the bulb - to do that you will need to remove it from its housing. You may need security drivers to get the outer case off.
You might consider leaving it without illumination and just open the door to see what's happening - or you might consider changing the bulb yourself or you might consider having a specialist call to do it for you.
How to change bulb of integrated Bosh Microwave HBC86Q620B ? ?
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