The 1830 compositor accidentally changed the modal must to might here in Jacob 2:6; must was restored in the 1908 RLDS edition (and later in the 1981 LDS edition) by reference to 𝓟. Obviously, must works much better than might, especially given the typical forcefulness of Jacob’s language.
Summary: Maintain the original modal auxiliary must in Jacob 2:6, the reading of the printer’s manuscript: “I must testify unto you”.