Your designs are automatically saved in your Zazzle account. Here are the steps to access your saved designs.

Step 1:  

Click on your profile icon in the header at the top of the site. (make sure you're logged in)

Step 2:  

On the menu click on the link "saved designs". 

Step 3:

Once you're in your saved designs, you can search for the design that you'd like to continue working on. If you'd like to delete a design, simply click on the checkbox next to the design(s) and click on the "delete selected" link above.