How to call the event ComboBox1_SelectedIndexChanged in c #

It is not invoked directly to the code of an event, it is not a good development practice, the reusable code should be taken to a separate function

public void ComboBox1_SelectedIndexChanged (object sender, ...)
{
    FuncionalidadComun();
}   

public void button1_Click(..)
{
    FuncionalidadComun();
}

private void FuncionalidadComun()
{
    //aqui codigo comun
}                

If from two places you need to invoke shared code, you take it to another function

Inside the function you could try the following:

ComboBox1_SelectedIndexChanged(null,null);

I hope I helped you. Greetings.