Let me give it a try - I think you're asking how to change the information on the About Us page, rather than the source code.
As the other poster said - you can do this by going to
1. Design >
2, Content
3. Then Select the About Us and click on edit
You will then be on the content page for About_Us
You can change the description
You can change the Content - you ca add graphics within this area.
I have made screen shots for you - There are in two posts due to size limitations.
hope this helps