I have problem on updating column set as primary key, even I made on update set default

this is my code for creating tables, I set on update set default

```
create table department(
id int default 10 primary key,
name varchar(50)
);

create table employee
(
id int primary key,
dept_id int default 40 foreign key references department(id) on update set default on delete set default,
    name varchar(40)
);

```

after that, I inserted data to the tables

```
insert into department
values
(1,'hr'),
(2,'programming'),
(3,'telesales'),
(4,'database')

insert into employee
values
(1,1,'mohammed'),
(2,2,'magd'),
(3,1,'soha'),
(4,3,'sameh'),
(5,4,'ashraf')

```

but, when I run this code to update the column id

```
update department
set id = 44 where id = 4

```

I get that error

```
The UPDATE statement conflicted with the FOREIGN KEY constraint "FK__employee__dept_i__571DF1D5". The conflict occurred in database "test", table "dbo.department", column 'id'.
The statement has been terminated.

```

but I do not know where is my fault!

thanks