Unless you have some kind of history table & trigger in place, to retain old values at every change, or you made a copy of the table before you ran the update, you will need to use the last backup that was taken before that update. Restore it (as some temp database) and extract the data.
You do have a backup available, right?
BTW, next time you're doing an update, I suggest you set it up like this:
-- UPDATE t SET Column1 = x, Column2 = y
FROM MyTable AS t
SELECT first to see exactly which rows will be updated. If needed, adjust the
WHERE clause to get the rows you want to target. Only then mark the sentence from the
UPDATE till the end and execute it. Much safer then running a "blind" update, as you did, forgetting the
WHERE clause (I assume that was the problem).
Even safer - do it on some test database first :).