Can I open/edit the existing(default) maps? And where are they?

Yes you can. here you can download them (text of the maps in polish though). To do it by yourself just do the following (from the same thread):

1) Extract the map from the data.pak file.
2) In this example we will convert multiplayer map named "M10", so firstly open extracted "\Maps\Multiplayer\M10\" folder.
3) Change the name of M10.xdb file to map.xdb.
4) In third line of map-tag.xdb file, change "M10.xdb" to "map.xdb" so the line looks like this:
Code:
<AdvMapDesc href="map.xdb#xpointer(/AdvMapDesc)"/>

5) Now in some other place create this folder structure: "\Maps\Multiplayer\M10\".
6) Copy all the map files to this folder structure.
7) Pack this structure into ZIP archive, (don't compress only store).
8) Change name of the file from .zip to .h5m.
9) copy this .h5m file to your Heroes V Maps directory.
10) Now you can open this map in the editor.