From these two places, you can access the editor.
From the dashboard
Simply click the edit icon, under the 'options' button in the bottom right hand corner of the quiz and you'll be returned back into the content editor.
From the content overview page
At the top right of the page you'll find the "edit" button.
Keep in mind that all changes you make to your content will automatically save all progress, but these changes won't be reflected to the published version until you press publish/update.