To remove formative assessment data you would go to My Subjects, select the required class and click the formative markbook, select the checkpoint the data was entered at and 'Choose'. In the top left hand corner of the screen click onto the red arrow and you will be in undo mode, now click onto any marks that are incorrect and each click will go back one mark. Once complete click onto the red arrow and you can continue entering marks as expected.
Please be advised that you can only remove marks at the checkpoint they were entered. If you are unsure where the marks were entered if you hover over the mark it will display the checkpoint it was entered at.
Please note: Formative data cannot be removed in bulk.