When you need to edit a job ad and make improvements, first find it, and then click the 'Edit job advert' link in the Page navigation menu on the left.
This will take you to the Edit page, which works the same as the Create page. You can change the job's title and the text. Clicking the 'Update job ad' button at the foot of the page to save your changes.
At the top right of the page there is a button ("Improve your score") that will display a list of 'good' words you can add to your job advert to improve its score, as well as an Info button that explains the colour highlighting used.
When you add new 'good' words (or 'bad' words!), they won't be highlighted until the advert is saved – you'll see them on the page when it reloads.