1. Usually, an agent upgrade would happen automatically if the credentials provided for agents under the "Manage agents" section are valid or has the appropriate rights for accessing services or logs in the agent machine.
   
2. However, in recent builds (after agent version >=4.1 or EventLog Analyzer version > 12130), the credentials are no longer required and, the agent uninstalls and re-installs automatically, along with the EventLog Analyzer service pack upgrade - provided the agent-server communication is intact.
   

Note: If you have initially deployed the agents manually or via GPO, you will need to uninstall and re-install the agents manually or via GPO. Please refer to the guide here for the for the Agent installation via GPO. Agent MSI file can be located in the below path:

1. <Install Home>\ManageEngine\EventLog Analyzer\lib\native\EventLogAgent.msi file