Here the typesetter for the 1837 edition replaced the preposition upon with on, which is what we expect in modern English. This change was probably not due to editing, especially since it was not marked by Joseph Smith in 𝓟; moreover, virtually all instances of “engrave upon plates” were retained in the 1837 edition. For further discussion of the phrase “to engrave (up)on plates”, see under Mosiah 28:11; in that instance, the 1837 edition made the same change as here in Mormon 1:4. The critical text will restore the original upon in both cases.
Summary: Restore the preposition upon in Mormon 1:4, the reading of the earliest textual sources (𝓟 and the 1830 edition).