All users have now been automatically upgraded to our new HTML content editor and will no longer be able to access or switch back to the old editor. When in the "Content" step of a campaign, you will only be able to see and use the new editor. (This change went into effect for all Internet Explorer users in June 2015 and all other users in April 2016.)
This change is required in order to allow us to take advantage of newer HTML5 technologies such as drag and drop functionality that will be a part of our upcoming releases.
The new editor has been redesigned from the ground up to make creating campaigns easier and faster, but if you experience any issues when using the new editor or have any feedback, please contact us.