you have to unzip the whole project, look for a file called project.c3proj and open that in a text editor, and on the second line of the file you will see:
change the number to the version you want to open it with... in this case I changed it to
save the file, re-zip the whole thing (and change the extension to .c3p)
NOTE!!!! this will only work if you have not started using new features not found in the version you are going back to... so make sure you keep lots of backups! and use at your own risk :) I have successfully done this many times with C2 - this is the first time I have tried this with C3, but it seemed to work fine.