Вот код из форума Unity:
//Import the following.
[DllImport("user32.dll", EntryPoint = "SetWindowText")]
public static extern bool SetWindowText(System.IntPtr hwnd, System.String lpString);
[DllImport("user32.dll", EntryPoint = "FindWindow")]
public static extern System.IntPtr FindWindow(System.String className, System.String windowName); //Get the window handle.
windowPtr = FindWindow(null, "Old Window Title"); //Set the title text using the window handle.
SetWindowText(windowPtr, "New Window Title - Yayyy");
Ссылка на форум Unity