You need to
set identity_insert YourTable ON
Then delete your row and reinsert it with different identity.
Once you have done the insert don't forget to turn identity_insert off
set identity_insert YourTable OFF
SET IDENTITY_INSERT Table ON
suppose we have to update identity value from 2 to 1
DELETE FROM Table WHERE ID=2
INSERT INTO Table(IdentityCol,) VALUES(1)
SET IDENTITY_INSERT Table OFF