You have to drop existing PK and create a new one.

>I have table as below with PK crefdoc+nsn, how could I change the PK to crefdoc+cbzunit using T-SQL?
>
>

CREATE TABLE [dbo].[fbzplnd] (
>	[crefdoc] [char] (15) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL ,
>	[nsn] [decimal](3, 0) NOT NULL ,
>	[cbzunit] [char] (6) COLLATE SQL_Latin1_General_CP1_CI_AS NOT NULL ,
>	[nvalue] [decimal](10, 2) NOT NULL
>) ON [PRIMARY]
>