Step 1. First, navigate to your Products page.
Step 2. Click on the Edit button next to the product.
Step 3. Click on ＋ Add Group followed, by Add Existing Group (or Create Modifier Group/Create Upsell if you are yet to make one).
Step 4. Select the modifier/upsell group(s). Clicking on the arrow will expand the group to show its contents. Click on Add.
The order of the groups can be changed by dragging and dropping them. This only changes the order for one product.