You likely found this article because images or other Shopify product data is not syncing with GroovePacker. The good news is, you're in the right place and this should be fairly painless. Once the process is underway you can continue reading about Why this step is necessary, (and you are also welcome to skip it).
The option to Re-Associate All Products With Shopify can be found by navigating to Settings and then clicking on the name of your Shopify store. The option is shown here:
There are two options below the button. I would go with the default option first (associate unconnected items) since it is much faster and will work 90% of the time. If you happen to know that the products in your GroovePacker account were all imported from Shopify directly (Not ShipStation, ShippingEasy, CSV, etc) and are not updating as expected, then, in that case, I would use the second option (Re-associated all items)
Here's the bad news. This operation will take a while. If you have thousands of products it's going to take a very long while. The reason it takes so long is that it is searching your Shopify account for each SKU and saving the Shopify product ID if it finds a match.
The first option ignores items that already have the Shopify ID saved, and focuses just on items with no ID at all. This can be a lot faster if nearly all of your items were imported directly from Shopify and already have the ID.
The second option ignores the ID (rather than assuming it is correct) and it requests the ID for every single item in your catalog. This is required only if the saved ID is wrong.
You may be wondering why the Shopify ID is necessary. When we use the Shopify API to update products the API does not use the SKU for identification, it uses the Shopify ID. This requires us to have the ID saved to properly receive product updates. When products are created using a Shopify import we are able to save the ID so we can use it for updates and all works smoothly. The problem arises when products are created by other sources that are not able to provide the ID. It's these cases where the option to re-associate items with Shopify becomes necessary.