If you are new to Varnish, here is what Wikipedia says about ti:
Varnish is an HTTP accelerator designed for content-heavy dynamic web sites. In contrast to other HTTP accelerators, such as Squid, which began life as a client-side cache, or Apache, which is primarily an origin server, Varnish was designed from the ground up as an HTTP accelerator.
Well today I want to show you how to purge all the cache without the need to restart the daemon.
varnishadm -T 127.0.0.1:6082 url.purge .
That will flush the cache.