Here we have two minor changes. First consider the omission, probably accidental, in the 1837 edition of the word again. The text just before has already referred to the Lamanites in the last days as being brought once again to the truth:
In Helaman 15:13, the again was restored to the RLDS text in 1908, to the LDS text in 1981.
There has been some tendency in the history of the text to accidentally omit again. Out of 18 more examples of this error, there are two that the 1837 typesetter was responsible for:
The second change here in Helaman 15:13 occurred later, in the 1852 LDS edition; in the first printing of that edition, the typesetter accidentally replaced the original modal shall with should. The correct shall was restored in the second printing for that edition, probably by reference to the 1840 edition. A similar replacement of shall with should occurred earlier in verse 9 of this chapter (but in the 1837 edition); see the nearby discussion under Helaman 15:9. Either reading, shall or should, will theoretically work here in verse 13. The critical text will follow the earliest reading, thus “they shall again be brought to the true knowledge”.
Summary: Maintain in Helaman 15:13 the reading of the earliest text: “they shall again be brought to the true knowledge” (with its use of the modal shall and the adverb again).