Once an alert is created, you can no longer edit its defining search. True or false?

False

Explanation:

There are various several to edit alerts. You can edit its defining search. You can view and edit alert details in Alerts page, you can review and perform additional edits to the alert by opening the detail page for the alert. Moreover you can view or modify the search string of an alert on the Search page.

Additionally, the authorized users can also review and update settings of an alert. They can edit permissions, alert actions, trigger conditions and can enable or disable an alert.

So you can edit defining search of an alert after it is created.