If all you're doing is updates, you can so this with an update trigger and a single update statement:

The trigger:

ALTER TRIGGER upd_PayrollEmployee

ON Payroll_employees

AFTER update

AS

IF COLUMNS_UPDATED() & 2 = 2

INSERT mylog (Date_of_change, Field, OldValue, NewValue)

SELECT GETDATE(), 'firstname', d.firstname, i.firstname

FROM inserted i JOIN deleted d on i.employee_code = d.employee_code

AND d.firstname <> i.firstname

IF COLUMNS_UPDATED() & 4 = 4

INSERT mylog (Date_of_change, Field, OldValue, NewValue)

SELECT GETDATE(), 'surname', d.surname, i.surname

FROM inserted i JOIN deleted d on i.employee_code = d.employee_code

AND d.surname <> i.surname

IF COLUMNS_UPDATED() & 8 = 8

INSERT mylog (Date_of_change, Field, OldValue, NewValue)

SELECT GETDATE(), 'department', d.department, i.department

FROM inserted i JOIN deleted d on i.employee_code = d.employee_code

AND d.department <> i.department

IF COLUMNS_UPDATED() & 16 = 16

INSERT mylog (Date_of_change, Field, OldValue, NewValue)

SELECT GETDATE(), 'email', d.email, i.email

FROM inserted i JOIN deleted d on i.employee_code = d.employee_code

AND d.email <> i.email

GO

The update:

UPDATE Payroll_employees SET

firstname = h.firstname, surname = h.surname, department =h.department , email=h.email

FROM HR_employees h

WHERE Payroll_employees.employee_code = h.employee_code

and (Payroll_employees.firstname <> h.firstname

OR Payroll_employees.surname <> h.surname

OR Payroll_employees.department <> h.department

OR Payroll_employees.email <> h.email)