The two manuscripts, 𝓞 and 𝓟, and the 1830 edition read “neither way”. The use of neither along with the preceding not creates a multiple negative (“the Lamanites could not retreat neither way”), which was removed in the 1837 edition by replacing neither with either. This change is in accord with standard English, but the critical text will restore the original multiple negative. For a general discussion of the use of multiple negatives in the original text, see under negation in volume 3.
Summary: Restore in Helaman 1:31 the original neither even though it results in nonstandard usage: “the Lamanites could not retreat neither way”; instances of multiple negation were quite prevalent in the original text.