We can start our required session only in case of sequential batch in case of concurrent batch we cant do like this.
Open previous runs of the workflow inside workflow monitor and right click on session and select "Restart task". This will run the session.
You can also use pmcmd command in UNIX to run a session.
You can open the workflow in Workflow Manager and execute the session by right click on it. You can use this option only when the session is not inside any worklet. The session should be present in main workflow only.