Another way to say "Replace your currently selected item"?

I'm building a part to our e-commerce store, it's essentially a "kit builder" for building several products in to one big kit, giving the user a discount.

The use can only select one item per category. If they decide they no longer want that item, and want to replace it with another item in that category, there's not way to tell them that the original item has been removed. Our buttons say "Add to Kit" and "Remove from Kit" on the currently selected item. So, if they hit "Add to kit" on another item in that category, it will remove the other item.

We essentially need another way to say "Replace the currently selected item in this category with this new item", but we need to say it in about 3 words. Something like "Replace current product", but more intuitive. Any ideas?