In your file system navigate to
steamapps\workshop\content\294100\2016436324\Textures\Things\Item\Resource\Steel

!!!BEFORE you start moving files, create a backup of the folders!!!
Make a new folder, call it Backup and copy the two folders you are about to manipulate into the new folder.

- Select the 3 pictures showing the old steel texture
- Cut them out by pressing ctrl + X, this greys the pictures out
- Go up one directory to the folder Resource
- Find the folder Old_Steel and opn the folder
- Paste the cut-out steel textures into this folder by pressing ctrl + V
- Select the 3 pictures of the new steel texture and transfer them into the Steel folder like you did with the old texture pictures
- Rename the pictures with the old steel texture to Old_Steel_ a b and c
- Rename the pictures with the new steel texture to Steel_a b and c

Check in-game if it changed the textures to the new ones.

Hope this helps