There must be a better way.
Clone the skin files to /storage/.kodi/addons/skin.myskin and edit the addon.xml to define a new/different skin name so there is no name clash. Restart Kodi for it to see the 'new' skin as something installed but disabled. Enable the skin. You can now edit the files in-situ using nano. You can reload the skin using a keyboard command (if one is attached) or use a kodi-send command to reload if editing 'remotely' over SSH (can't remember the command, Google will find it for you).