To edit or delete a project which has been posted (but not assigned to a freelancer yet), you'll first have to visit the "My Projects" page. Navigate to the "Posted Projects" tab and open the project you'd like to edit by clicking on its banner.

Under the Intro Box (the box on top), you'll see two icons on the right side - a blue pencil icon and an orange trash icon. Edit the post by clicking the pencil; delete the post by clicking the trash.