On a community home page, there are viewlets that contain content to be displayed to users of the community. To update this content, use RAMHomepageViewlet.setContent(String).
//Update the Content of a Homepage Viewlet
viewlet = community.getHomepageViewlet("My Viewlet");
viewlet.setContent("New Content for My Viewlet");
session.put(community, new NullProgressMonitor());