Go to Products > Product Groups.

Click Edit next to the product group that you need to change.

You should see setting for that product group, a section to add products, and the list of products in that group. You can delete any of those products from the group to remove them.

There are two ways to add products to the group.

1. Browse for the product using the category tree. Then click add when you have found and selected a product.
2. Add products by sku. Check the “Add products by SKUs” checkbox. Then enter the sku’s (comma separated) in the text box that appears.

After you have made your changes, click on Save.

Then generate custom pages (or entire site).