I found this code on the old forums

It allows you to edit the
  - Window icon
 - Task Bar icon
-  Task Manager icon

1. void setup1()
   {
   
   
    
     PGraphics icon = createGraphics(16, 16, JAVA2D);
     icon.beginDraw();
     icon.noStroke();
     icon.fill(#55AAFF);
     icon.ellipse(8, 8, 16, 16);
     icon.fill(#FFEE22);
     icon.ellipse(9, 6, 8, 6);
     icon.stroke(#FFEE22);
     icon.strokeWeight(3);
     icon.line(6, 6, 6, 12);
     icon.endDraw();
     frame.setIconImage(icon.image);
   }