I know of two ways. Both involve the use of an audio editor program, also called a Digital Audio Workstation or DAW. The one I use is called Audacity. First, see if you can find a “multi-track master” version of the song. This is the form the recording is in before it is “mixed down” to the normal two (stereo) channels. Then you can just open the song’s tracks all in your DAW, delete the vocal track(s), and mix it down yourself. The second method is more difficult and less satisfactory. It only works on recordings in which the vocals are identical on both left and right tracks, and where the instrumentation is not (that is, a given instrument is all on one side or the other, or at least uneven.) These sorts of recordings are pretty rare, but if your song is of this type then you can open it in your DAW and simply invert one of the tracks. This will make the parts of the two tracks that are identical “cancel” each other out. Parts that are similar but not identical will become faint and distorted. Or you can just purchase a “karaoke” version of the song from any one of many, many vendors of such things.