Create a .env file in the root of your project directory, and store the variable MESSAGE_STYLE=uppercase in it.Then, in the /json GET route handler you created in the last challenge access process.env.MESSAGE_STYLE and transform the response object's message to uppercase if the variable equals uppercase. The response object should either be {"message": "Hello json"} or {"message": "HELLO JSON"}, depending on the MESSAGE_STYLE value. Note that you must read the value of process.env.MESSAGE_STYLE inside the route handler, not outside of it, due to the way our tests run.If you are working locally, you will need the dotenv package. It loads environment variables from your .env file into process.env. The dotenv package has already been installed, and is in your project's package.json file. At the top of your myApp.js file, add require('dotenv').config() to load the environment variables.

Sure, here are the steps to follow:

1. First, create a .env file in the root of your project directory. You can do this using any text editor, or from the command line using the touch command like so: touch .env.

2. Open the .env file and add the following line to it: MESSAGE_STYLE=uppercase. Save and close the file.

3. Now, in your myApp.js file, you need to require the dotenv package at the top of the file. Add this line to do so: require('dotenv').config().

4. In the /json GET route handler you created in the last challenge, you need to access the MESSAGE_STYLE environment variable and use it to determine the case of the response message. Here's how you can do it:

app.get("/json", function(req, res) {
  let message = "Hello json";
  if (process.env.MESSAGE_STYLE === "uppercase") {
    message = message.toUpperCase();
  }
  res.json({message: message});
});

In this code, we first set message to "Hello json". Then, we check if process.env.MESSAGE_STYLE equals "uppercase". If it does, we transform message to uppercase. Finally, we send a JSON response with message.

5. Save your changes and run your server to test it. Depending on the value of MESSAGE_STYLE in your .env file, you should see either {"message": "Hello json"} or {"message": "HELLO JSON"} when you visit http://localhost:3000/json in your web browser.

Remember, the dotenv package allows us to load environment variables from our .env file into process.env, so we can use them in our application.