SQL Serever 2008 ALTER TABLE Problem

hi,
i had this problem when i was trying to modify the identity of a a field in a table Named TestTable,i tried to modify it from the object explorer but it didn't Save my changes :
(Saving changes is not permitted,etc)
i tried to write a sql statement to alter my table but it is not working...
Satement i am using :

ALTER

ANY Help ???

TABLE TestTable ALTER COLUMN id IDENTITY (1,1);