Frameset.ChildFramesetItem (Word)

Returns the Frameset object that represents the child Frameset object specified by the Index argument.

This property applies only to Frameset objects of type wdFramesetTypeFrameset. For more information on creating frames pages, see Creating Frames Pages.

ChildFramesetItem (Index)

Index: The index number of the specified frame.

ActiveWindow.Document.Frameset _ 
 .ChildFramesetItem(3).FrameName = "BottomFrame"