I accidentally pressed the rewind button (at the bottom of the camera) of Nikon FM2.
I was at my 15th exposure.
In such situation, what should I do?
Will it destroy my whole exposures?
The rewind button on manual cameras serves to release the take up spool so that you can use the rewind lever to get the film back into that can. Pushing the button alone doesn’t do anything. Releasing it puts things back how they were.
Side note, on cameras that don’t have multiple exposure support out of the box, you can hold the button down while cranking the film advance lever...this causing the film not to move (because of the release button being held) but the shutter still cocked (allowing you to take a second exposure on the same frame).
Some cameras rewind including the leader. In that case, you need a "leader retriever" (don't know the proper name) or a darkroom (and we are talking pitch black dark here, not the kind of red light you can use for work with photographic paper) to fix it. Of course, you then need to shoot 15 (or better 16) completely black photographs (short exposure while leaving the lens cap on) before continuing and hope that the camera aligns them correctly with the already shot photographs so that automated cutting of negatives does not make a hash of the rest. Personally, I'd likely not bother for a half-shot film and just let it get developed as-is (you won't get billed for or even get prints from the unexposed negatives, so you are just wasting a bit of film and negative development cost).