If you've been using git push heroku main to deploy your code, you can pull it down using the heroku git:clone command displayed in the Deploy section of the dashboard for your app. After pulling down your code, you can follow the instructions to push to your destination of choice.
If you want to take advantage of Heroku's Github integration, you can move your source code to GitHub by pushing your code to a Github repository and linking it with Heroku.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more info related to the ending of free plans.