If it is publish instance you want resume rerun upgrade I would advise to start from beginning. If it is author add [1] to start up or follow [2]. After upgrade completes remove [1]/[2] which ever you followed. More details how it works refer http://dev.day.com/content/ddc/en/gems/upgrading-to-56.html
[1]
-Dorg.apache.sling.launchpad.startupmode=update
[2]
* From crx explorer create/set the /var/upgrade/status/upgradesExecutor.forceUpgrades property to true.
* Stop and restart the com.day.cq.cq-upgrades-executor bundle from felix console