Here Oliver Cowdery initially wrote the preposition on as upon in 𝓟. Virtually immediately he corrected the upon to on by crossing out the up (there is no change in the level of ink flow for the crossout). Oliver was probably prompted to write upon here in verse 26 because of its two occurrences in the phrase “lay hands (up)on X” earlier in this passage:
Overall the Book of Mormon text is fairly equally divided between “lay hands upon X” and “lay hands on X” (13 to 12). There is a passage in Mosiah 13 where there is variation in the use of upon and on for this phrase:
So either reading is possible, and we follow the earliest textual sources in each case. For a general list of mix-ups of upon and on in the textual history, see under Alma 2:38.
Summary: Accept in Helaman 5:26 Oliver Cowdery’s correction in 𝓟 of upon to on (“ye cannot lay your hands on us to slay us”).