The autocomplete list is stored inside your project’s .scrivx file, which will be called YourProjectName.scrivx, and stored inside the YourProjectName.scriv project folder.
See this post for editing guidance.
Before you proceed, remember - the .scrivx file is the index (heart) of your project. L&L does not endorse users poking around inside it. If you bork it, your project is borked. I suggest making a copy of your project folder and experimenting on that first. Regardless, make full project backups before proceeding. Ensure that you have something to fall back on if things go south.
Best,
Jim