I have a plugin I modified and I want to download a copy of it because it is different than the original version available for download from the plugin store. Is there a way I can do that?
You can use FTP to get these files. Plugins are stored in 'wp-content/plugins' directory. All you have to know is the name of its directory/file.
PS. You should never modify original plugins. You will loose ability to update such plugin and updates are crucial for security.