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 dungeon 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 > Characters > Creatures > Dungeon
Drag and drop the downloaded files here, overwrite if asked to.

Compatibility
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Might not be compatible with mods that modify the dungeon creatures, especially the shadow witch.

Uninstall
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Possibilities:
1. Replace the modified data.pak file (or data folder) with the original.
2. Reinstall the game.
3. If on steam, repairing the game should replace the modded files with the original ones.