Using Sigil is like baking a cake, you assemble the ingredients (html/stylesheets/images/fonts) and output a cake (epub), when you made a cake, you can't usually then get the ingredients back.
But if you really want the individual html (that has been altered to work IN the epub) you can open the epub with a zip program and extract the relevant files, any html etc will be altered to have any links etc. relevant to other files as though it was in a structured epub, NOT as if it was a flat html.
Does that make sense?
Saving as html from Sigil is irrelevant and would cause further bug/error reports which aren't Sigil's fault because the files are to be used INSIDE an epub.
If you want backups, just store a copy of the WHOLE epub somewhere else, then none of the individual components can't get lost/overwritten.
It made sense when I was writing it
