There are two ways you can get to 5.8.4
1. Upgrade the agents using the Agent Commands feature in the UI.
Go to the platform and navigate to the Hyperic Agent resource > Views tab > Agent Commands.
For upgrading more than one agent, create a Compatible Group of Hyperic Agents then in the Group properties you should be able to upgrade the entire group of agents at once.
2. Update the agent files in vIN.
This KB has the steps for updating the agent files that are stored on the vIN appliance.
VMware KB: Upgrading the Hyperic agent in vRealize Infrastructure Navigator 5.8.2