What version are you running? AE will open most previous versions but if you have a project that was saved from a newer version it will not open in older versions of After Effects. You need to get access to the newer version and then use the Save As feature to save the project as an older version. Let's say the project was created in CC 2014 (or 2015 the only two versions that are interchangeable) and you wanted to go back to CC. You would save the File as a CC copy. If you wanted to then go back to CS6 you would have to open the file in CC and do the save as again to save the file as a CS6 copy. You'll just have to go through this routine until you get all the way back to the version you are using.
The other thing that might be happening is that you are misreading the warning. Maybe the project file was created in an earlier version of AE and AE is simply warning you that the file is being converted to version 10. This does not cause a problem and is normal. You just won't be able to save it back to an earlier version of AE unless you go through the steps I listed above.
i have cs6
The error message ought to say something along the lines of, "I'm converting this file from an earlier version". That's fine. If it asks your permission to convert it, say yes. If it says something else, you've got a problem... one which may be solved by just running an update on CS6.