Yes, you can change the design of a Shopify product which has been already connected to Gelato.
1. Click on Stores and select the store the product is in
2. Find the product from the list and click on Edit next to the product
3. Find the variant you want to edit and click on Edit next to it
4. Follow the flow to reconfigure the product and upload the new design.
Note: Depending on whether the variant was connected using Gelato online editor (Edit) or by Upload your print functionality (Upload new print file), you will see different ways to rework the variant.