You can't open one form in another form's window. The best you can do is to use the button to HIDE the one form and OPEN the other. It looks about the same from the user's perspective.
Code is "doCmd.OpenForm <form name>......." (intellisense will take you the rest of the way.)
Alternative:
Tab Control with subforms, but use invisible tabs. Put a subform on each tab and use your button to switch tabs programatically.
Code:
Create Tab control (tab1) with however many pages (Page1, Page2, Page3, etc.) Set Tab COntrol's STYLE to NONE.
Button can switch tabs with "Page3.SetFocus" or "Page2.SetFocus", etc.
Kirby