Commiting / Overwriting changes to CVS

I am using WinCVS on Windows. I made some changes to my source code which is in .net and committed the changes to cvs by doing Update Selection / Commit Selection. I have created multiple versions on cvs. Now I want to overwrite the entire code which is there in cvs with some very old version of the code which is lying in some branch? What should be the correct way to so. The code in the branch and main trunk differ to a large extent.

Any help on this is highly appreciated.

Thanks,
Vandana