During the Aseret Yemei Teshuva ("Ten Days of Repentance," from Rosh Hashana through Yom Kippur), Halacha requires that in place of the Beracha "Ha'Kel Ha'kadosh" in the Amida service, we recite instead "Ha'Melech Ha'kadosh."
If a person forgot to make this substitution, and he did not catch his mistake until after the period of "Toch Kedei Dibur" (1-2 seconds) passed since he recited "Ha'Kel Ha'kadosh," he must return to the beginning of the Amida. If he forgets to the make the substitution the second time, then he must again return to the beginning of the Amida, until he recites the proper text of "Ha'Melech Ha'kadosh." If one catches his mistake within "Toch Kedei Dibur" of reciting "Ha'Kel Ha'kadosh," he simply recites "Ha'Melech Ha'kadosh" and then continues the Amida as usual.
This Halacha applies as well to the "Beracha Me'ein Sheva" recited by the Chazan during the Friday night service after the Amida. In this Beracha, too, the phrase "Ha'Kel Ha'kadosh" is changed to "Ha'Melech Ha'kadosh" during the Aseret Yemei Teshuva. The Chazan should pay close attention to ensure to make this substitution, and the congregation should listen attentively so they can correct the Chazan if he forgets. If the Chazan forgot to recite "Ha'Melech Ha'kadosh" and he catches his mistake before concluding the Beracha, he returns to that point in the Beracha, corrects his mistake, and then continues as usual.
If the Chazan did not catch his mistake until after he completed the Beracha, must be repeat the Beracha Me'ein Sheva?
This point is subject to a debate among the authorities. The Chid"a (Rabbi Chayim Yosef David Azulai, Israel, 1724-1806) and the Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909) ruled that the status of Beracha Me'ein Sheva is the same as that of the Amida in this regard. Just as an individual who forgets to recite "Ha'Melech Ha'kadosh" in the Amida must repeat the Amida, so must a Chazan repeat the Beracha Me'ein Sheva if he mistakenly recited "Ha'Kel Kadosh."
Others, however, including the Peri Chadash (Rabbi Chizkiya Da Silva, Italy-Israel, 1659-1698) and the Peri Megadim (Rabbi Yosef Teomim, Poland-Germany, 1727-1792), held that a Chazan is not required to repeat the Beracha Me'ein Sheva if he neglected to recite "Ha'Melech Ha'kadosh." This is the position adopted as the final Halachic ruling by Chacham Ovadia Yosef. (Torat HaMoadim)
Summary: A person who forgot to substitute "Ha'Kel Ha'Kadosh" with "Ha'Melech Ha'kadosh" in the Amida during the Aseret Yemei Teshuva must return to the beginning of the Amida and recite the proper text. However, if on Friday night during the Aseret Yemei Teshuva a Chazan forgot to make this substitution during the Beracha Me'ein Sheva, he need not repeat the Beracha.
