You can mass-edit a particular piece of a product via the X-Cart admin.
In Apperance options, set the max number of products to display in admin as higher than the total number of products in your store.
Click on 'Modify Product' in admin, and leave the search box empty. Hit Search. It should show you ALL of your products.
Click 'Check all' and then 'Modify Selected'
It will show you a product with checkboxes next to each piece. Check the box next to the field you want to be the same for all products, edit the field and hit save.
Be VERY careful with this method as you can easily botch up your entire store
