You can get to this by going to https://leanpub.com/p/PUBLISHER/settings where PUBLISHER is your publisher url slug.
Near the bottom is a section for "Landing Page Blurb". Once you've filled in your text, click the blue "Update Publisher" button at the bottom of the page.
You can also navigate there by signing in with the publisher account, clicking the arrow next to your avatar at the top right, selecting "Dashboard" under "Publisher", clicking on the blue "Dashboard" button for the publisher, and then (whew!) clicking Settings.