Switch Data Gateway to other Dataset

Hi. I want to switch or change data gateway from 'Out of Stock Report' dataset to 'Sales Movement Report'. But, failed. 

What I tried before:

- I clicked on "Discard" button and that Data Gateway still tie to this dataset.

- I removed 'Out of Stock Report' dataset and that Data Gateway can't be viewed or selected by other dataset too.

How to switch it?

Thanks.