table : abc

no (primary key) name
================ ====
1 jung
2 love
4 tom


In this record, I wanted to update no 4 to 3 which is primary key.
So, I used the query as below.
=============================
Set Identity_insert abc on

UPDATE abc
SET no = 3
WHERE no = 4

Set Identity_insert abc off
go
=============================
But I got an error;
Server: Msg 8106, Level 16, State 1, Line 1
Table 'abc' does not have the identity property. Cannot perform SET operation.

Why this happens? I really want to update the primary key.