How do I reverse the affect of the sensor value? I tried swapping the 'HIGH' and "LOW' but that don't work.
sensorValue = analogRead(sensorPin); // read the value from the sensor
digitalWrite(ledPin, HIGH); // turn the ledPin on
delay(sensorValue); // stop the program for some time
digitalWrite(ledPin, LOW); // turn the ledPin off
All the sensorValue does is set the speed of the flashing. You can't "reverse" that. Negative times don't exist.
You've given almost no information but if there is a potentiometer connected to "sensorPin" that is setting that value and what "reverse" means to you is "change the direction you turn the pot for faster flashing" then you should swap over the +5V and Ground connections to the potentiometer.
If you really need to do it in code try
sensorValue = (1023 - analogRead(sensorPin));