It would depend on when the instillation fails. If you upgrade the application monitoring server to a new version and it fails then you can roll back with no impact. If you upgrade the collectors and they fail you can roll that back without any issues as long as none of the agents restarted and communicated with the collector to pull the new version (agents upgrade from collectors). You could also have done a delayed agent upgrade by adding the appropriate debug flag and then those agents would not upgade to the new version until you specify them to.
So the short answer is that it depends when/how you did your upgrade for if and how a rollback will need to be done.