It depends (which should be the answer to everything). Basically, you really can't "unmix" something that has been mixed. Mostly. BUT, there are a few options to try.
One is seeing if you can get an instrumental track of the tune in-question. I mean, the exact song, the exact cut, the exact length, with no vocals (in other words, the opposite of an acapella). What you are hearing, when you hear these home-made acapella tracks, is usually someone that has sourced a sans-vox mix, and then inverted it and blended this with the original track, which causes everything to phase-cancel out (except the vox). There are plenty of tutorials on the process on YouTube and elsewhere on the Web. Without the instrumental mix though, pretty tough.
Another way would be to use the Thompson Vocal Eliminator (a hardware device, but I think there is now a plug-in version?). This uses stereo center-cancelling to get rid of the lead vocal (mostly), which can help you create the instrumental mix you'd need to do the phase-cancel trick above. Lather, rinse, repeat.
Lastly, there is relatively new software that can be used to boost or attenuate drums in a mix (it works surprisingly well, from what I've seen and heard). It's called, interestingly enough, "Unmix: Drums."
zynaptiq: UNMIX: DRUMS
There are of course, a plethora of EQ's, notch filters, multiband compressors, etc., etc. to play with, but under normal circumstances you are going to be mostly frustrated by the process.
Do some Googling and God Speed...