Ok, it's the moment to interact with the deployed contract. First of all, we have to know that every interaction is a promise, so asynchronic. Let's open the console:

truffle console

Inside the console, we can get a contract reference.

var helloWorldContract;
HelloWorld.deployed().then(function(contract){ helloWorldContract = contract; });

With this reference let's get the default greeting, calling the getGreeting function.

helloWorldContract.getGreeting.call().then(console.log)

Now let's change the greeting and get it again.

helloWorldContract.setGreeting("Hola mundo inteligente!").then(console.log)
helloWorldContract.getGreeting.call().then(console.log)

Great! :) Next module consists on deploying the same contract but on a RSK node.