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h5u is an archive format. That is, you can take any zip file and rename it to the h5u format; it's not complicated.
You have a new texture. Go to the hero file, look at the path to the .Model file. In the properties of this file, look at the paths to the materials in the materials folder.
You need to create copies of the files for this model and materials. Open the mod archive and make copies of the specified files. I hope I don't need to explain how to duplicate files.
Give the new copies a clear name or simply add a "2" at the end.
In the map editor, for this hero, in the Character properties, enter the path to the copy of the .Model file (Model2). If the path was inserted and remains in the line, it means you entered everything correctly; click the three dots, and the properties of the model file will open. In the list of materials, enter the path to the copies of the material files.
Then, for each material, click the three dots, and the material properties will open. In the Texture field, click New, enter the name of the new texture; a texture properties window will open. Set Texture Type to DXT3, ConvertType to CONVERT_ORDINARY, REGULAR, CLAP. In SrcFile, select the texture in tga format from the Complete folder of your game (if there is no folder, create it and put the texture there).
Then, set NMips to 2.
On the name of the texture file in the file tree (on the left in the properties window), right-click => Export.
When the export is finished, click OK everywhere.
Repeat the operation with the remaining materials if there are more than one.