1. Navigate to the product page to add Up-sell / Cross-sell function, inside theme customization.
2. Under sections, click on the Upsell settings.
3. From here you are able to edit the design, and add products or collections to upsell.
4. Click on "Upsell Product" to select the product you'd like to show the pop up on.
5. Click on "Upsell Collection" to select which product(s) to show on the pop up. You can create a collection with single product to Upsell. Add up to 3 products to a collection and use it to Cross-sell.