You can adjust the frame rate where you want in your sketch, which means you can call the frameRate() method in setup() or draw(), depending on what you want to do.

Just put it in the setup() if you want it to be fixed :

void setup() {
 frameRate(30);
}

If you want it to change, say depending on the mouse's position, you can put it in the draw() method :

void draw() {
 frameRate(mouseX);
 background(random(255));
}

Nicolas.