There are a couple of screws to remove the housing & clean (some alcohol on button) the camera button. (gonna assume you got some gunk on it)
Google 'Lumia 920 Service Manual' - this will provide instructions & show the schematics / diagram of what you're doing.
Alternatively, (somewhat riskier) get some alcohol on a Q-tip & attempt to infuse some around the camera button. Given time, this should free it up. If you attempt this method, I'd still check out the service manual.
Last option, a cellphone repair shop should be able to remove the housing & clean that area ... question is cost.