If on Motzei Yom Kippur one said “zachreinu…” and he did not yet finish the beracha, he goes back to the beginning of the beracha.
If he finished the berachah, he should go back to the beginning of shemoneh esrei.
If he said “uchesov…” or “besefer…” (and finished the beracha), he goes back to retzei,
If he said hamelech hakodosh (or hamelech hamishpat), if he realizes it immediately, he should correct himself. If not (1.5 seconds later), continue.