Almost any WAV editor will let you convert a file from stereo to MONO. There are a lot of free ones out there if you search on google for "Free WAV editor"...
Wavosaur is a good one.
WavePad is another good one, but it also asks you to upgrade to the non-free version and it's kind of annoying, so I'd use Wavosaur.
Wavosaur:
http://www.wavosaur.com/download.php
Direct-download:
http://www.wavosaur.com/download/files/Wavosaur.1.0.1.0(en).zip
Then open your file with it, press CTRL+A to select the whole WAV, go to the Process menu, then Convert to Mono, then 50% 50%