Customer wants to download code from old version of the app using rollaback and github.
Using dash board or CLI command
heroku releases -a app-name
see all the releases and find the version number you want the code/slug to be downloaded.
Then, You can rollback to the version for the deploy commit, which will create a new release/version and then you can download the latest slug.
Also, Whenever possible you should simply revert the relevant code changes locally with git revert and redeploy.
example : $ git reset --hard 6f1a5b4d or $ git reset --soft 6f1a5b4d