To remove a product from your offer, first click the "Select product manually" button.
Search for the product that you want to remove by typing its name in the text box provided.
Once the product appears Uncheck the product that you want to remove from the offer.
Then select the product that you want to be displayed on the offer hit the "Save" button.
Select either Save Draft or Publish to lock in your modifications. Doing so will ensure your offer no longer contains the previously added product.