Thanks for your message, I would be happy to help you out with this. We actually have a handy help guide that you can check out here, that walks you through different options for doing just this. However, I would be happy to outline them for you here as well:
From your Shopify admin, go toProducts>Collections.
Click a collection.
In theProductssection, click the drop-down list besideSort, and then select a sort order.
If you selectManually, then you can click and drag the products in the list to reorder them. This will then change your default collection image to whatever product you switch to. You can also click and select one or more products in the list. ClickMoveto reorder them to a specific position in the collection.
I hope this helps you out moving forward, please feel free to reach back out here with any questions, we are always happy to help out!
Marty | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog