How can I release the oven switch?

Well, you COULD have done a Google image search for the item. That may have given some clues.

For instance; within 60 seconds I found this:

http://www.ebay.co.uk/itm/NEW-Genuine-Oven-Selector-Switch-32016037-for-Bush-Logik-Beko-Servis-AE6BFS-/222155325030

.....which suggests there are two M4 screws that go in from the front of the chassis.

You will probably have to work put how to remove the front panel.

XXX
December 2016