To continue working on a level you have previously saved, you can find it in the Level Loader and open it in Free Play. When you save it again, it will be a new copy of the level, and it will need a different name.
You can delete your old Local Levels with the Delete File button, so you can get rid of earlier versions of your levels.
You can also open one of your online levels in Free Play and save it again. However, once a level has been put online, it cannot be deleted. You can follow the instructions on this page to hide Unpublished online levels from view.