As discussed under Alma 8:4, preach and teach are used rather interchangeably (and sometimes together) in the Book of Mormon text, so in each case we follow the earliest textual sources. Here we have an instance where the original preach was accidentally replaced by teach in the 1888 LDS edition. No subsequent LDS edition used the 1888 edition as copytext; thus the error here was never perpetuated.
Summary: Maintain in Alma 23:3 the verb preach rather than teach, a typo in the 1888 LDS edition.