Java agents cannot be restarted through the AppMon client. Instead, restart the Java Process (JVM) that your agent is attached to - an agent is alive only as long as the monitored process is alive.
If you require a restart in order to apply instrumentation changes, consider a Hot Sensor Placement. This allows the agent to apply the changes without having to restart the whole process. You can do this on eligible agents from the Agents Overview dashlet by right clicking on the agent and choosing "Hot Sensor Placement".