CREATE TRIGGER lastUserChange on users_table

FOR INSERT, UPDATE

AS

DECLARE @user_id int

SELECT @user_id = (SELECT id FROM inserted)

UPDATE users_table SET last_change = getdate() WHERE id = @user_id

Now whoever is updating any field on specific row in users_table, last_change gets updated.

P.S. This works only when you change one row at the time. To change that, you need one subquery and bit of imagination;)