the link you have is good explaining how to create and deploy the function app from VS Code. Given that you have an existing function app running with some python, I would recommend the following:
- Download the content, project and settings of the existing function app (from the Azure portal)
- Run the project locally and ensure it runs successfully. You need to check the Functions runtime used and ensure you have the Azure functions extension installed on your VS Code
- Do the changes you need, test them and once your are happy deploy them to your Azure function app on Azure using the Azure extension and the Deploy to Function App option.
Hope this helps you out. Let me know what are the results.
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