Right-click on the Google logo and inspect the HTML in the dev tools. Look for IDs, class names, etc.
Open up the JavaScript console in the dev tools (CMD-option-I on Mac).
Using document method, find the Google logo and store it in a variable.
Modify the source of the logo IMG so that it's a Yahoo logo instead.
Using document method, find the Google search button and store it in a variable.
Modify the text of the button so that it says "Yahooo!" instead.
var img = document.getElementById('hplogo');
img.width = 400;
img.src = 'http://www.logostage.com/logos/yahoo.GIF';
var button = document.getElementById('gbqfba');
button.innerHTML = 'Yahoooo!';
Part 2: The Unsuspecting Website
Open up a website (maybe a favorite of yours? or an anti-favorite?).
Take a screenshot of what it looks like before.
Using document methods in the console, change the webpage. Change images, text, etc.
You may want to periodicially copy your code into a separate file, so that you don't lose it
if you accidentally reload the page.
Take a screenshot of what it looks like after the transformation.