You just want to take the AppImage apart, alter it and convert it to an AppImage type 2 if I’m reading correctly?
Type 1 AppImages are ISO disk images, so you should be able to mount or extract them using the same tools that you would for normal CD/DVD images. To build it into a type 2 AppImage, you need to download AppImageTool (available here: Release Continuous Build · probonopd/go-appimage · GitHub) and then to use it, run ‘ARCH=x86_64 ./appimagetool… [extracted type 1 directory]’