You don’t need to change a lot of similar arts in different places into the project. Just change one art in Textures_png folder, and this art will replaced in all places of the project, where it should be located.
Open a project
Go to Assets → Textures_png
For reskin, you should to change all arts in all folders into Textures_png Folder.
EXAMPLE, open Textures _png Folder --> Items (folder includes all items and blocks images in the project). Replace your arts in Textures_png --> Items to reskin all images in this Folder. Do the same for all folders inside the Textures_png.
Remember - your art should be the same size and only in the PNG format like in the project, to correctly display the textures in the project.