Step 1:

Make sure that you're on Step 3: Edit Content. You may be drafting your email for the first time, going back to Step 3 during the same session or re-visiting your draft campaign. You may also have selected the "past emails sent" tab and need to edit for am up-to-date campaign.

Step 2:

You can edit headings, sub-headings, replace images and edit a text box.
To edit a text box click on the pencil icon:(shown below)

 

Step 3:

Whilst you're in a text box you will see the text editor toolbar:

The toolbar gives you access to great functionality to enhance your campaigns including link to documents (add file icon on extreme left), forward to a friend (quick links drop-down), social integration (custom tags > add social links, which inserts %social% into the text box), add/edit links (to webpages, email address)
Once you're happy with your text box, click on Save button.
You have successfully edited your campaign and may now be ready to select recipients (step 4) and test & send (step 5)