To modify an existing multi-row query, open the settings view for the query part and select Edit on page 1 of the Query Spec notebook tab.

Selecting Edit opens the SELECT Details window on the query. "Use the SELECT Details window" describes how to use this window.

You can also manually edit an query, but if you do so, you can no longer use the SELECT Details window to edit it. To manually edit a query, turn to page 2 of the Query Spec notebook tab and select Manual edit.