It is not possible to amend the description or pack sizing on an existing product because of tracking reasons, you would need to delist and re-build it.
To do delist follow the below steps:
Search for product via product locator.
Once the product appears, click on Pricing tab.
Click on Delist Product button.
This will remove the product from the portal for all customers, to re-add follow the below steps below
Click on Product Management tab on left hand menu.
Click on Single Product Creation tab.
Re-add product back into the system with correct details.
This will also allow you to use the same SKU code.