What program do you already have, and/or already familiar with? On some programs I think it's referred to as "mid side decoding".
Audacity is free and has a plugin for mid/side, but rather than switching to another program (which has a learning curve) you may be better off doing it with your existing program, even if you have to do it "the hard way" without a plugin. If you have used your editor to mix AUD + SBD together you have the skills to do Mid/Side easy enough. Basically the right channel is what you get when you mix the MID + SIDE together, and then you make the left channel is what you get when you INVERT the side channel and mix MID + INVERTED SIDE channel together. I know it sounds confusing, and maybe I'm the only one who does it this way... but then I am able to tune in the WIDTH of the mix by adjusting the volume of the SIDE channels and they come into the mix. To me that is easier than guessing at ratios.