Be warned this can induce errors if your project is using newer features not supported by the older version of C2. That being said:
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 (I recommend notepad++, get it here).
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, r184 will look like 18400). Change the first 3 digits to match version you want to revert to.
6. Save File. If you want to go back to a capx, don't re-zip it; just open the project in C2 and resave as a new capx. DON'T SAVE OVER YOUR OLD FILES WHEN DOING ANY OF THIS.