The Master Schedule will lock down as soon as attendance has begun being recorded for that day. To correct this you will need to remove any attendance associated to sections for that date and the Master Schedule will be able to be modified. 

To remove the Attendance: 
  1.  Navigate to Academics > Scheduling > Class attendance > Section Attendance.
  2.  Set the Attendance Filter to Recorded 
  3.  Click the "Clear" button for all recorded attendance sections
This will clear out the attendance records; However, the attendance will still be preserved in the section. Once attendance has been cleared and the Master Schedule has been updated you can re-record the attendance taken by following the same steps and clicking "Take Attendance" and then clicking Save & Close.