I haven't personally got one for Replace Assets (yet), but I can advise on it.
Firstly, you will need to drag the name of the asset you want to replace into the Assets to Replace part of the box, then drag the name of the asset you want to replace it with into the Replace With box.
Make sure This Section Only is unselected (i.e. not in yellow/orange), then click Replace. If there's a lot of track to replace, it may take a minute or two.
Shane