You've changed your mind and want to show a different static product in the content block. That is possible, even when you already changed the design.
First you click on a random element of the product block, such as the title, description, image or button. You then see the layers of the section that this product block is in. Click on the product icon (Switch to row) to go back to the product settings.
Now, to select a different product, you again click on Static to view the pop-up with all synced products. Choose the different product and click on OK. The other product will now be shown in your message.
How to show a different product in the old messages editor?
If you want to show a different product in the content block, you can double click on the content block. You will then see an overview of synced products of your webshop. Here you can choose the product you want to show.