Access your Triggers from "Settings" on the left-hand side menu. There you'll see a list with your created triggers. Hover over the preview you want to edit and click on it.
When you do this, the trigger window will open to change whatever settings you want
Once you finish editing remember to save, otherwise your changes will not be applied.
Note: If you have previously created text ads, you can only delete them, not edit them. This is only until the ability to create text triggers is enabled on the platform.