how to clear upstream proxy cache?

Hi,

There is a version of a file on my webserver
that is somewhere other than in my cache.
It appears to be upstream, which is to say
on a what I gather is a proxy server,
somewhere between my client system and my
webserver.

Can someone tell me, how do I tell that
server to let go of my file? Merely cleaning
out my cache & doing a browser reload has no effect.

Thanks!