nGrinder supports easy upgrade. Actually you don't need any additional step except just replacing nGrinder binaries. They upgrade the database automatically and use the ${NGRINDER_HOME}/.ngrinder folder content continuosly.
Nothing is really necessary to upgrade controller. Just replace ngrinder-controller-{VERSION}.war in the your tomcat webapps folder. When you restart the tomcat, nGrinder detects the current database schema version and try to upgrade it to the version you're installing. Just it works.
If you're upgrading controller, mostly previous version agents are just working with later version of nGrinder controller if the special announcement is not commented. If you'd like to upgrade the agent, just replace the current agent binary with newer version. And run run_agent.sh or run_agent.bat.