As noted under 1 Nephi 3:2, the archaic in the which has sometimes been replaced in the text with in which. In this instance, the definite article the was omitted in three early 20th-century LDS editions (1902, 1905, and 1911). The 1920 LDS edition restored the the, undoubtedly by reference to the earlier text. For a general discussion, see in the which in volume 3.
Summary: Maintain the original in the which in Jacob 5:17.