You cannot move an email from one campaign to another. What you can do is save the email you would like to to move to another campaign as a template and choose that template in the campaign you need it. Only afterwards you can delete the email from the first campaign, from the View Emails:
To save your email as a template, you need to go to the email editor and in the Settings tab and click the Save as template button below.
This will save your email into the templates selector page (on the bottom of the page)