The homepage content that is editable will be controlled under the CMS section of the admin.
Navigate to CMS > PAGES > MANAGE CONTENT
On the left, you will see a page tree. "Home" should be selected by default
In the center of the screen, select the "Content" tab.
You will by default see the coding for the homepage. From here you have two options.
1. Scroll down until you see the placeholder text (approximately line 47) and cut, copy and paste your new text in.
2. In the left-hand corner of the editor box, click the "Show/Hide Editor" button. This will replace the code with the graphics and formatted text. Scroll down until you see the "About Us" section and enter the new text in.
Once done editing the content, be sure to click Save Page.
The last step will be to refresh the cache so that your changes are reflected on the frontend of your website.
Now you'll want to flush cache:
SYSTEM > CACHE MANAGEMENT. Select Flush Cache Storage at the top right of the page.