in order to edit or remove the end date from an FSM period, go to the profile of the student in question. On their extended tab, scroll down to the Further Information section.
Look for the Manage button as pictured below:
Click on this and you will see the information on the student's FSM period.
To edit or delete the end date entered, click on the Edit button (take a note of the start date) and delete the entry. Next, re-create the FSM period here, entering the start date only, and click Save.