Using the included UV map as a reference, feel free to paint it with your favorite image editing software and export it as a .png or .jpeg file to the Texture folder.
Open the steam library.
Download and run AnimazeEditor. (Type “Animaze” in the search field and it will come up.)
In the upper left corner, go to Assets -> Import 3D Object -> Select the .fbx file you downloaded.
You will see a lot of warning windows, press OK or X to clear all the windows.
Click on the part of the model you want to change to select it, and when it turns green, open the “Diffuse Texture” at the bottom of the “Editor” tab on the right, click on the square icon, and select the image you created. Click on the square icon to select the image you created (if it doesn’t appear, drag and drop the image).
Select Save All Materials, Save Item, Bundle in that order, and save the bundled .avatar file in an easily recognizable location.
↑Please refer to here to import the .avatar file you have just created and use it.