I asked for .NET C# as the backend while generating my application code, but got node.js instead.

To switch your application's backend from Node.js to .NET C#, you'll need to replace the Node.js server code with a .NET C# backend framework like ASP.NET Core. Start by creating a new ASP.NET Core project in Visual Studio or via CLI, then re-implement your API endpoints, logic, and database connections. Update any frontend API calls to match the new .NET endpoints. If you have specific requirements or existing data models, you can reuse them within the C# backend to ease the transition.