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I presume that you mean the Shutter Button is what you have a replacement for and would like to replace.
It might perhaps snap-fit into place. It depends upon how the original one came off in the first place - if a plastic snap-tab broke off, the button might no longer be retained in place and get lost or not be usable. So even if the new button snaps into place, be careful as the new one might break if roughly put back onto the camera.
Be sure that you have the right replacement button, of course... a wrong replacement might both break, and/or break its mounting on/in the camera. And if the camera mounting of the shutter button is broken, just replacing the button won't be the cure for the problem, unfortunately.