The law if a mistake was made during the blessing of an Aliyah:

The law if one accidentally said the after blessing first:[1]

If prior to the reading, one accidently recited the blessing of Asher Nasan Lanu instead of Bachar Banu, then if he did not mention Hashem’s name in the concluding stanza of the blessing, he is to go back and recite the correct words of Asher Bachar Banu. If, however he already said Hashem’s name in the blessing, then he is to conclude “Nosein Hatorah” and he fulfills his obligation. In such a case, by the after blessing he is to recite the blessing of Asher Bachar Banu. [If in such a case he did not say the blessing of Asher Bachar Banu, but rather repeated again the blessing of Asher Nasan, then if he already said Hashem’s name in the blessing, he is not to repeat the blessing of Asher Bachar Banu.[2]]

The law if one accidentally said the before blessing instead of the after blessing:[3]

If after the reading, one accidently recited the blessing of Asher Bachar Banu instead of Nasan Lanu, then if he did not mention Hashem’s name in the concluding stanza of the blessing, he is to go back and recite the correct words of Asher Nasan Lanu. If, however he already said Hashem’s name in the blessing [i.e. Baruch Ata Hashem] and remembered right away, then he is to immediately continue “Elokeinu Melech Haolam Asher Nasan Lanu.” If, however, he already concluded the blessing then he is to repeat the after blessing of Asher Nasan Lanu from the beginning.

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[1] M”A 139:5 in name of Beir Sheva p. 58, Sotah 41b; P”M 139 A”A 5 “Safek Brachos Lihakel”; Derech Hachaim; M”B 139:15; Ketzos Hashulchan 25:7; See Piskeiy Teshuvos 139:11

Other opinions: Some Poskim rule that if one recited the after blessing instead of the before blessing, then he is not Yotzei and must repeat the before blessing. [Maharil Hilchos Kerias Hatorah p.452, brought and negated in in M”A ibid; Elya Raba 140:3; Shaar Efraim; Magen Giborim] See Piskeiy Teshuvos 139:11

[2] Piskeiy Teshuvos ibid footnote 66

[3] Derech Hachaim; M”B 139:15; Ketzos Hashulchan 25:12; See Piskeiy Teshuvos 139:11

Other opinions: Some Poskim rule that if one recited the before blessing instead of the after blessing, then he is Yotzei and is not to repeat the after blessing. [Shaareiy Efraim 4:23; Toras Chaim Sofer 139:6; Misgeres Hashulchan 2; Kaf Hachaim 139:49; Az Nidbaru 5:30; Mishneh Halachos 8:17]