You can edit your website info section (Which you see on the top tray bar) from Admin -> Settings -> Site Wall Page -> About Us.
Note: This page also has permissions settings which apply to your "Website Activity" wall module as well. These options are:
- Who can create posts
- Who can delete posts and comments
Since the "Site Info" section and the "Activity Wall" Module are essentially the same feature (But applied in different ways), these permission will apply on both areas.