You can use Sigil to edit the ePub directly, and add the new chapter to the end, but that's perhaps a bit complicated. You'd have to
* view page source to get the HTML, * create a new XHTML chapter in Sigil (that's easy; you right-click the "text" folder & select "create new"), * copy-paste the "head" section from the previous chapter into the new one, * copy-paste the HTML text from the new chapter into the "body" section of the new chapter in Sigil.
Otherwise, you can re-download and just save over the file you already have. I don't know if that will let you skip past chapters you've already read, but the chapters are bookmarked; if you remember what chapter you left off on, you should be able to navigate to the new one. And it's possible that if the file contents are the same, you'll just jump to the page you left off on.
ETA: I'm assuming you're talking epubs of fic archived at AO3. I don't know exactly how it works if you're using fanfic downloader for (example) ff.net files; the principles are similar but I don't know if the internal structure is the same.
no subject
* view page source to get the HTML,
* create a new XHTML chapter in Sigil (that's easy; you right-click the "text" folder & select "create new"),
* copy-paste the "head" section from the previous chapter into the new one,
* copy-paste the HTML text from the new chapter into the "body" section of the new chapter in Sigil.
Otherwise, you can re-download and just save over the file you already have. I don't know if that will let you skip past chapters you've already read, but the chapters are bookmarked; if you remember what chapter you left off on, you should be able to navigate to the new one. And it's possible that if the file contents are the same, you'll just jump to the page you left off on.
ETA: I'm assuming you're talking epubs of fic archived at AO3. I don't know exactly how it works if you're using fanfic downloader for (example) ff.net files; the principles are similar but I don't know if the internal structure is the same.