Oliver Cowdery first wrote to as he copied the text from 𝓞 into 𝓟; then later in somewhat heavier ink flow he corrected the to to unto (perhaps while checking copy against the original manuscript, no longer extant here). The correction here in 𝓟 is clearly Oliver’s, unlike the insertion of they in verse 6. The use of the preposition unto is unexpected in modern English, so the correction here probably represents the reading of the original manuscript.
The Book of Mormon text has only one example of “to the end of X”, in 1 Nephi 20:20 where Isaiah 48:20 is being quoted: “utter to the end of the earth”. Otherwise, the Book of Mormon text has “unto the end of X”:
Summary: Follow in 2 Nephi 27:10 Oliver Cowdery’s correction of to to unto in 𝓟 since unto probably represents the reading of the original manuscript.