If you need to make an adjustment to the design being sent in an automated campaign you can do so quickly and easily.
Head to the specific active automated campaign that you would like to change the design on.
You can only edit the design of automated campaigns that have been stopped. Here's a specific article on how you can stop an active automated campaign.
Now that your automated campaign is stopped you'll see a new button that will allow you to edit the design of that campaign. Click the "Edit Design" button.
On the next page you'll want to choose which new/updated design you'd like to use for this campaign. Once you've done that click update and confirm your choice.
Once you've switched out the design you'll just need to restart your campaign.