Generally the effect of slow-motion is produced by filming at a higher frame-rate and displaying the movie in a lower frame-rate. For instance, to get 2x slow-motion, you could record in 50fps and playback in 25fps.
You are saying that the slow-motion videos you have are in 23, 25, 29 fps. This is the playback rate. Originally they were recorded in higher frames-rates that are unknown to us. But we can try to restore the original speed by displaying more frames per second or by cutting out frames and see if it looks realistic. I had a look around and I could not find a what standard slow-motion frame-rates are. If you cannot find out either you will have to guess.
You can use ffmpeg
to modify the framerate of your videos as described here https://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video . If you wanted to double the video playback speed (to restore from 2x slow-motion), you can do:
ffmpeg -i input.mkv -filter:v "setpts=0.5*PTS" output.mkv
But I would recommend reading the short article in the link above to understand what this and the alternative commands are doing.