Update or Modify operation alters the existing values. If these operations alter the value that is the foreign key also, the only check need is the similar as that of the Insert operation.
What should happen to an try to update a candidate key that is the objective of a foreign key reference? For instance, an attempt to update the PROJID "LG" for which there exists at least one matching ASSIGNMENT tuple? In common there are the similar possibilities as for DELETE operation:
RESTRICT: The update operation is "restricted" to the case where there are not matching assignment tuples. (it is rejected otherwise).
CASCADE - The update operation "cascades" to update the foreign key in those matching assignment tuples also.