alter table Project drop constraint DF_ID
alter table Project add constraint [新约束名] primary key([列名])

------解决方案--------------------

竟然没一个会的……
   DECLARE @Constraint_Name varchar (200) 
   
   select @Constraint_Name = Name  
   from dbo.sysobjects  
   where Xtype = 'PK'  
         and Parent_Obj = (select [ID]  
                                 from dbo.sysobjects  
                                 where id = object_id(N'admin')  
        and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
   
   EXEC ('alter table admin drop constraint '+@Constraint_Name )