Oliver Cowdery initially wrote “unto Ether” in the printer’s manuscript, but later he corrected unto to to by crossing out the un (the level of ink flow for the crossout appears to be somewhat heavier, like the other corrections on this page of 𝓟 that were the result of Oliver’s proofing of 𝓟 against 𝓞). Elsewhere, when referring to the word(s) of the Lord coming to someone, the text has four examples with the preposition unto but three with to:
Since either preposition is possible, it seems unlikely that the corrected reading in 𝓟 for Ether 13:20 is due to conscious editing; rather, the original manuscript probably read “the word of the Lord came to Ether”.
Summary: Maintain in Ether 13:20 the preposition to in “the word of the Lord came to Ether”, the correction in 𝓟 (which very likely reflects the reading in 𝓞).