When you click on sent newsletters you get a list of all the previously sent newsletters. Sent newsletters can be viewed as a history that shows which newsletter has been sent. Therefore you cannot delete or edit a newsletter that has already been sent.
On the other hand you can quickly restore a newsletter you have sent earlier. To restore you merely click on the button restore.
The icon looks like this: 
When you click on restore you will generally just be sent to create newsletter where the fields are already filled in with the information from the previous newsletter.