Chazan accidentally skipped Birchas Kohanim:^[1]

Remembered in middle of Sim Shalom: If the Chazan accidentally skipped Birchas Kohanim and remembered in middle of Sim Shalom, then he is to go back and recite Birchas Kohanim, and then restart again from the beginning of Sim Shalom. This applies both to areas which perform Nesias Kapayim with a Kohen and the Chazan ended up skipping it, and to areas that don’t perform Nesias Kapayim and simply recite Elokeinu in its stead. In areas which perform Nesias Kapayim with a Kohen and the Chazan ended up skipping it, remembering in middle of Sim Shalom, he should go back and have Nesias Kapayim performed and then restart again from Sim Shalom. In areas that don’t perform Nesias Kapayim and simply recite Elokeinu in its stead, then the Chazan is to go back and say Elokeinu and then restart from the beginning of Sim Shalom.

Remembered after concluding the blessing of Sim Shalom: In areas that Nesias Kapayim is not performed and simply the paragraph of Elokeinu was skipped, then they fulfill their obligation and do not need to repeat Chazaras Hashatz. In areas that perform Nesias Kapayim, then if they remembered after concluding Sim Shalom, Nesias Kapayim is to be performed without a blessing at that moment.

[1] Yalkut Yosef p. 316; Piskeiy Teshuvos 127:3; 128:33; See Meiri Megillah 24b [Not Chozer]; Halachos Ketanos 2:306 and Shalmei Tzibur [return to Ritzei if finished Sim Shalom]; Toras Chaim Sofer 128:32; P”M 128 M”Z 35