You cannot update an existing aggregate, nor overwrite it or re-use it's name.
You must first delete the existing aggregate though the AGGREGATES tab of the TABLES screen.
To delete an aggregate, follow these steps:
From there you must find the Aggregate name that you wish delete. If you hover over it, a checkbox will appear:
Or you have the option of selecting all:
Once you have the aggregate selected, a trash can button will appear which you must click on to remove the selected aggregate.
A confirmation window will appear, giving you the final chance to confirm or cancel the delete operation.