To unassign the fee to students, follow the path given:
Finance-->Fee Management-->Assign Fee to Students-->select Academic Year,Branch,Class,Section,Regular fee type(select fee type which you
want to unassign),student segments,category,Type of student-->click on Get students-->To unassign,select Assigned Students option-->click on
check box for the respective student to whom you want to Unassign the fee-->click on Unassign fee to selected students.