Open a copy of index.js. If you haven't already downloaded the code for this project, you can find a copy of index.js here on GitHub. You can use a simple, lightweight code editor like Atom, Sublime Text, or VSCode, but you also have the option to edit the code directly in your Lambda function.
I wasn't sure the appropriate place to submit this, but the link to the custom index.js in the customization section of the developer.amazon.com version of the tutorial is incorrect. Right now it links to https://developer.amazon.com/alexa-skills-kit/lambda/custom/index.js but it should link to https://github.com/alexa/skill-sample-nodejs-city-guide/blob/master/lambda/custom/index.js .
It's in the paragraph that reads: