int i= 1;
if (i == 1){
this.Icon = WindowsFormsApplication1.Properties.Resources.Icon1;
i = 2}
else
{this.Icon = WindowsFormsApplication1.Properties.Resources.Icon2;
i = 1;}
This way u can change the icons.
To test this code, simply create a form and add a button. In the button's click event add the provided code.
then create 2 icons in the project and run.