Follow this link. You'll see a page with your saved searches. Find the search you want to modify and click (or tap) on the Settings button. In Settings, you can only modify the alert frequency of a saved search. To modify your criteria, load the search results for the saved search by clicking the View Search Results button, adjust your criteria as you see fit, and then click Save Search with the new criteria. Then go back to your Saved Searches and delete the original one.