Go to Campaigns > Email.
Select either the Sent or Unsent tab, as appropriate.
Select the checkbox for the campaign you want to copy.
Expand the MORE ACTIONS drop-down menu.
Select Copy campaign
Give the copied campaign a new name, and choose which folder you'd like to put the new campaign in from the dropdown (you can always change these later on if you want to).
Select COPY.
The copied campaign will appear, as newly named, in the selected campaign folder location under the Unsent tab. From here, you can edit it.