Here the 1837 edition accidentally changed the preposition from unto to to. Elsewhere in the Book of Mormon text we have six occurrences of “to preach repentance unto someone”, but none of “to preach repentance to someone”:
The preferred preposition in this construction is definitely unto. The critical text will restore the original preposition unto here in Alma 16:13.
Summary: Restore in Alma 16:13 the original preposition unto in “preaching repentance unto the people”; elsewhere the text consistently has the preposition unto in the construction “to preach repentance unto someone”.