Yes, you can change the details of a Squarespace product that has been already connected to Gelato.
Editing the Design
From the Stores section, select your Squarespace store and find the product you wish to make changes to. Select the correct product and click on the pencil button. The design editor will load and you will be able to adjust the design.
Be sure to click on continue when you're happy with the changes for them to be reflected in your store.
Editing the product title and description
From the Stores section, select your Squarespace store and find the product you wish to make changes to. Select the correct product and click on the blue Edit details button. You will be able to make changes to the product title and description. Simply click Publish to Store when you're happy with the changes and the product will be updated in Squarespace.
Edit the price
You can modify the retail price of your product directly in your Squarespace store.
You can see how this is done by checking out the article Editing Products in Squarespace's Help Center.
Edit Media and Images
It is not currently possible to add new mockups from Gelato to Squarespace for already published products. However, you can upload new and update existing images to your product when editing a product in Squarespace. You can read more about how to do this in the article Product images in Squarespace Help Center.
You can download mockups by selecting the product in your store products page in Gelato and opening the design editor to edit the design. From the design editor, you can call up the preview and download the mockup images you fancy by right-clicking on them and saving them to your computer.
You can then upload them in Squarespace.