I've taken apart and reassembled a mode dial on a Canon Rebel T3, so they definitely usually can be serviced. However, if you're planning on replacing it, makes sure that you are able to find a replacement mode dial. Alternatively you can also just replace the entire top cover of the camera, which would also make fixing the issue easier. Fortunately, it appears like you'd be able to disconnect all the cables connected to the top cover without the need for soldering (which isn't always the case).