When you log in, you will automatically be taken to your dashboard. While this has a variety of items to draw your attention, you are looking for "Campaigns", which is where the current and all past eblasts are.

The one at the top of the list is usually the most current one, but it never hurts to double check that.   Click on the third label at the top of the list and select "Last Modified", which will bring the most recent ones to the top. 

Once you have the most recent eblasts at the top, find the one you want to work on.  At the right on the same row, click on "Edit".