You may change your Guild Description through the Widget Customization mode - click 'Widgets' at the top of your guild site, then click the 'Edit' link that appears under the Guild Description widget.

More information on how that process works can be found here.

This was originally in the old admin system; when sites transitioned to the New Admin functionality, it was changed over to the widget area to streamline the process (all widgets are edited site-side).