After GitBook has synced content with your GitHub fork, you can use GitBook to make edits in your fork. At the top left of the GitBook window, you will see the name of the branch/version you are currently reading. If you expand that drop down, a list of available versions will be shown. The version "1.0.0" is the "master" version. (You can click on a version and change its name if you wish.) Select the one version edit. When it appears, at the bottom right, click the button to edit the selected version. The list of versions should show a link with the name "Create a new release +".