To add a new product to Shopify add a new row to the Airtable sheet where your products are being synced to. You can fill out as many of the columns as you need, but the minimum information required is the product name. Please note that when adding a new product you should leave the Shopify Id column blank.
After you are happy with the new product row initiate the Airtable to Shopify sync.
To update a product simply find the row that corresponds to the product you want to update in your Airtable sheet where your products are being synced to. Find the appropriate column you want to change and make the modification.
When you are ready to make the update to Shopify just initiate the Airtable to Shopify sync.