It's in the /cache directory, you'll need a root enabled file explorer to get it, like es file explorer. Unfortunately, if it's failing OTA, it will fail on a sideload. It is failing because you made some change to the system folder, i.e. deleted some bloat.
I can help you get updated, you'll need to install safestrap 3.11, and make a backup of your stock side install. Then you can replace the system image in the backup wit a clean one, I'll get you the link