How to update or insert an ID using a stored procedure in table

Environment: Win7 and SQL server 2008 R@

Tools: SQL management tool 2008 R2

T-SQL code - works fine- :

DECLARE	@return_value int, @new_id char(9)
EXEC	@return_value = [dbo].[proc_gen_id]

@new_id = @new_id OUTPUT
SELECT	@new_id as N'@new_id'

Outcome - for example-: 433194976 or 907959111

T-SQL code

CREATE TABLE employees
( 
  id char(9),
  employee_number number(10) not null,
  employee_name varchar2(50) not null,
  salary number(6),
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);

INSERT INTO employees (employee_number, employee_name, salary) VALUES (1001, 'Sally Johnson', 32000); 
INSERT INTO employees (employee_number, employee_name, salary) VALUES (1002, 'Will Smith', 38000); 
INSERT INTO employees (employee_number, employee_name, salary) VALUES (1003, 'Jessica simpson', 32000); 
INSERT INTO employees (employee_number, employee_name, salary) VALUES (1004, 'Michael Lira', 65000); 

Problem:

How I would update an id column, which is generated/called by the stored procedure

My code

DECLARE	@return_value int,
		@new_id char(9)

EXEC	@return_value = [dbo].[proc_gen_id]

@new_id = @new_id OUTPUT

SELECT	@new_id as N'@new_id'


UPDATE Employee Set id=@new_id 

Before

('null',1001, 'Sally Johnson', 32000);
('null',1002, 'Will Smith', 38000);
('null',1003, 'Jessica simpson', 32000);
('null',1004, 'Michael Lira', 65000);

After:Expected results

('433194976',1001, 'Sally Johnson', 32000);
('907959111',1002, 'Will Smith', 38000);
('819753931',1003, 'Jessica simpson', 32000);
('000753931',1004, 'Michael Lira', 65000);

Please help