How can I change the gram weight for a certain volume of an item already in the database?

You can edit the "Yields/Measures" area for ESHA ingredients and your own ingredients. I would advise (if you want to add one to an ESHA item), that you open that item, choose "Save As" and save it as a new item. That will leave the existing ESHA item unchanged, then you can edit the newly created copy as you wish. The reason is that if we update the item, it may "stomp" those changes and you would lose them.