Install
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Navigate to your Heroes V data folder. For me it is:
F:\Steam\steamapps\common\Heroes of Might and Magic 5 Tribes of the East\data
With the basegame navigate to: Heroes of Might and Magic 5\data

To be safe you should backup the data.pak file, or the data folder to another location.
Or you could backup the original files with the same name in the Succubus folder. Your choice.
This way you can return to the original state if you need to.

When in the data folder start by opening data.pak with WinRAR. This will allow you to view it's contents.
You will see a bunch of folders. Navigate to > Textures > Creatures > Inferno > Succubus
Drag and drop the downloaded files here, overwrite if asked to.