This has been covered numerous times and is found easily via search.
This will only work as long as you haven't made use of any new features after r206, obviously.
1. Go to the project file. If it's a .caporj file, skip step 2.
2. If it's a capx, unzip it (capx are just zip files basically).
3. Now, open the caproj file in a plain text editor.
4. Look for the following string, minus quotes: "<saved-with-version>". It's only a few lines down from the top.
5. You'll see this tag has a 5 digit number; it's the c2 version padded with 2 zeros (so, r212 will look like 21200). Change the first 3 digits to match version you want to revert to, i.e. 20600.
7. Double click .caproj to open within C2