There has been considerable variation in the history of the text between the prepositions upon and on (for a list of examples, see under 1 Nephi 12:4). Here in Alma 3:14, the 1837 edition replaced the archaic upon with on, probably accidentally since elsewhere the text uses only upon when referring to a mark being set “(up)on someone” and none of these instances of upon have ever been changed to on. All the other examples are in this same chapter of Alma:
Clearly the use of upon is correct in verse 14. The 1908 RLDS text restored the original preposition, as will the critical text.
Summary: Restore the original preposition upon in Alma 3:14, thus making the entire text consistent for the phrase “to set a mark upon someone”.