How can I delete a flight plan already filed?
You can’t. You can, however, file a new flight plan, which will overwrite your old one (if it isn’t locked).
And a flightplan gets “locked” when you are already online as pilot and an air traffic controller has made changes to your flightplan (transponder code, departure procedure, route change etc.).
So, if you have not connected to VATSIM yet and/or nobody has made changes to it, you can easily refile for the same callsign.
Also, an unused flight plan will automatically be deleted from your account after about two hours.
Log off Vpilot , Xpilot etc and it will let you make changes only if not already talking to atc.