In Windows, apparently Blizzard is recommending that you do a Find Folders and Files (Windows key + F) for "wowtest," and delete everything that pops up. On the Mac, everything is neat and tidy (as usual)-- it's all in the /Applications/World of Warcraft/WoWTest folder. Delete that, and you're good to go for the 2.3 PTRs.
I don't know if this is really required-- it seems to me that the WoW PTR Client would likely just overwrite anything whatever you had in those folders, and your PTR download would still work. But you never know-- maybe files will be left behind if you don't delete, so it's probably best to clear everything out and start over with the new download (and I bet it'll be a big one-- this is a big content patch). Plus, you can feel like you're all ready for patch 2.3: bring on the new PTR!