There are many ways of doings this, it depends how your forms are shown.
Are they in a ViewStack or similar container? If so just change the view from the click handler function for the button.
If they are in a Canvas/HBox/VBox, in the click handler function of the button you can do a check like if Form_A.visible==true then submit it and hide it, else submit the other form. You can also have a variable that tracks if Form_A was submitted.
If you have code, post it here and we can find the best solution with a clear example in front.
If you want this form to be shown as an popup object then you can use TitleWindow.
var ttlWin:TitleWindow = new TitleWindow();
ttlWin = PopUpManager.createPopUp(this, Form_A, true) as TitleWindow;
// You can take reference of ttlWin and set its properties.
ttlWin.x = 100;
ttlWin.y = 100;
Thank all !
I solved my problem by using Pop-up object.
Again, thanks very much.