Select the 'Runs' page in the Analyze Directory navigation panel. This will list the runs for all schedules. If required you can filter the list using the All / In Progress / Failed buttons at the top of the main panel.
Select the data flow that you want to restart. Then in the properties panel, click the Data Flow link - which will open the scheduled run as a read-only document.
Click on the 'Continue Run' button and then the 'Continue' button in the pop-up dialog box.
Note that the data flow can only continue if the error condition has been resolved, e.g. where the error was due to missing dependent files or network connectivity issues.
You can use the 'Edit Data Flow' button to make changes to the data flow itself before re-running it. Note any changes made will affect future runs of the data flow and also how the state information for other historic runs is displayed.