I'm using ganymede but I can't find the option to change mirror for the update sites. Is there a way to change this?
The all notion of mirrors is no more a first class citizen of eclipse Ganymede update mechanism, but rather a simple property of a repositories (for metadata or artifact).
You may still force a mirror for a given feature, with a command-line similar to the one mentioned in this bug.
eclipsec -verbose -application org.eclipse.equinox.p2.metadata.repository.mirrorApplication
-source http://www.someurl.com/ -destination file:x:/local/path/to/mirror
(not tested though)
See also this introduction to P2