I want to download code from an older version of the app.
Determine the release version number you want the code/slug to be downloaded from. The CLI command
heroku releases -a <app_name>will show you all releases for your app.
heroku slugs -a <app_name>to find the slug ID for the particular version selected. You'll need to install the Heroku Slugs CLI Plugin first if you haven't already.
heroku slugs:download <slug_id> -a <app_name>.