if(currentMillis - previousMillis > interval) {
previousMillis = currentMillis;
}
This bit is correct - but since you are not doing anything when the interval elapses it's a bit like having a clock and not bothering to look at it.
You need to include within the IF statement the stuff you want to happen when the interval elapses.
And, as @AWOL says, you don't need the delay() function at all.
...R