you can call INSERTED
, SQL Server uses these tables to capture the data of the modified row before and after the event occurs.I assume in your table the name of the key is Id
I think the following code can help you
CREATE TRIGGER [dbo].[after_update]
ON [dbo].[MYTABLE]
AFTER UPDATE
AS
BEGIN
UPDATE dbo.[MYTABLE]
SET dbo.[MYTABLE].CHANGED_ON = GETDATE(),
dbo.[MYTABLE].CHANGED_BY = USER_NAME(USER_ID())
FROM INSERTED
WHERE INSERTED.Id = dbo.[MYTABLE].[Id]
END