To retrieve a list of the content you have authored, see View personally authored content
Editfrom the actions box.
Editlink will take you to the edit form for your particular piece of content you wish to edit:
Body Textarea/File Uploadedit form (Documents)
Lead-in/Bodyedit form (News Item)
File Uploadedit form (Files, Images)
Simpleedit form (Link, Favorite, Event)
Enter the changes you wish to make to the content in the form or browse to changed version of the content on your filesystem as appropriate.
Viewlink from the actions box and view your content with the changes in place.