The printer’s manuscript, here the earliest extant source, has the singular word; the 1830 compositor (accidentally, it would seem) set the plural words. Elsewhere the text has instances of both thy word and thy words, two of which refer to deity:
the Lord’s a person’s thy word
1
2
thy words
1
8
The statistics are more distinguishing for the phrase my word(s), with 12 examples of the singular my word and 18 of the plural my words in referring to deity; for a complete discussion, see under 1 Nephi 16:24. In one nearby example (Helaman 10:4), the Lord, in speaking to Nephi, declares, “for I have beheld how thou hast with unwearyingness declared the word which I have given unto thee unto this people”, which uses the same singular word in referring to the Lord’s word as here in Helaman 11:16: “thou canst bless them according to thy word which thou hast said”. The critical text will follow the earliest extant reading here in Helaman 11:16, namely, “according to thy word”. For lists of other places in the text where word and words have been mixed up, see under Alma 35:3, Alma 37:20, and Alma 42:31. Also see under Mosiah 20:21 for discussion regarding the phrase “the word(s) of the Lord”.
Summary: Restore in Helaman 11:16 the singular word since it is the reading of the earliest extant source (here the printer’s manuscript); in general, the text allows either the singular word or the plural words in referring to the word(s) of the Lord.