I don’t that that is very easy to do at the moment, not sure what would be easiest. I think you can specify a different bundle url, but not sure if a sufficiently up to date bundle is available already. You could also try to just put the updated package files in the cache directory, but that’s really hacky and not sure it works.
Try raising an issue about how to update specific packages on github (https://github.com/tectonic-typesetting/tectonic/issues) maybe someone has a better suggestion (@pkgw )