Updating Locally Modified

[Prashant]

Hi, this is my problem.  I have a file that is locally modified (i.e.
newer than the version in the repository), but what I want to do
update my working version from the repository version (i.e. merge the
changes i've made).  CVS won't let me do this because it doesn't merge
locally modified files.  The reason I want to do this is because I've
downloaded some open source software, and made some modifications to
it.  I've committed the modifications i've made to a repository on my
local machine.  But whenever a new version of the open source code
comes out, I just want to download the files, and patch them with my
modifications.  Whats the easiest way to do this?