Here in the printer’s manuscript, Joseph Smith apparently intended to change “in the which” to “by which”. He crossed out the definite article the and supralinearly inserted by, but he forgot to delete the in. The 1837 edition ended up deleting the the but left unchanged the preposition in. As explained under Ether 10:10, Joseph changed four other cases of “in the which” to “by which” in the book of Ether. In each instance, the critical text will restore the original phraseology, “in the which”.
Summary: Restore in Ether 11:15 the original phrase “in the which”; in this case, Joseph Smith apparently intended to emend this instance to “by which”, but the 1837 edition ended up with “in which”.