int screenNumber = 0;

final int HOMESCREEN = 0;
final int MENUSCREEN = 1;
final int INSTRUCTIONSCREEN = 2;

void setup() {
  
}

void draw() {
 if (screenNumber == HOMESCREEN) {
  // Draw homescreen
  drawHomescreen();
 } else if (screenNumber == MENUSCREEN) {
  // Draw menuscreen 
  
  
 }
 
  
}

void drawHomescreen() {
  // do all homescreen drawing here
}

void mousePressed() {
 if (screenNumber == HOMESCREEN) {
   screenNumber = MENUSCREEN;
 }  
}