1) From Rosh Hashanah until Yom Kippur there are four additions in the Shemone Esrei: “Zachreinu LeChaim” (Remember us for life etc.), “Mi Chamocha” (Who is like You etc.), “UChsov LeChaim” (And inscribe for Life etc.) and “BeSefer Chaim” (In the book of Life etc.). If one omitted any of the above, but remembered before he said “Baruch Attah Hashem”, he must return and say it. If he had already said “Hashem”, he does not go back. (Shulchan Aruch and Mishne Berura 582:5)

2) Although we have said that he does not go back, nevertheless if one forgot “Zachreinu”, one inserts it in the blessing of “Shomea Tefilla” and if one forgot “UChsov” or “BeSefer”, one should insert it immediately before “Yiheyu LeRatzon” at the end of the Tefilla. On Shabbos, when Shomea Tefilla is not said, one also inserts “Zachreinu” before “Yiheyu LeRatzon”. However, “Mi Chamocha” is not inserted in either “Shomea Tefilla” or before “Yiheyu LeRatzon”, as it is a praise not a request. (Kaf HaChaim ibid and Ben Ish Chai)