If you already have a live site and wish to change the design, the process is simple. No migrations or domain changes are necessary, you only need to adjust the Site Settings in your new design.
Publish a New Design Without A Blog
First, make sure you are in the new design that you would like to publish in place of your old design. Next head to the Site Tab in the (left) Site Panel and choose Site Settings.
Next, choose to EDIT next to Custom Domain where it reads "Domain not set" to set your domain in the new design.
Once that has been set, close the site settings menu and hit publish in the top right corner of the Design App and your new design will be published and can be found at your domain!
Publish a New Design With A Blog
If you do have a blog, make sure you've completed the above steps and then read on below to complete the additional step required for your new design to be published.
For our blog users, you will next need to click over to the blog tab found in the Site Settings.
Once in the blog tab, choose your domain and then close the site settings and hit publish in the top right corner of the Design App! And just like that, your new design is published and live at your domain!