On the p2 director level, there is no 'update' operation. Instead, you need to tell the director to both 'uninstall' and 'install' the IU that you want to be updated:
eclipse -application org.eclipse.equinox.p2.director -uninstallIU org.nodeclipse.enide.maven.feature.feature.group -installIU org.nodeclipse.enide.maven.feature.feature.group -repository ...