Here the typesetter for the 1841 British edition accidentally replaced the preposition unto with upon. He was undoubtedly influenced by the preceding occurrence of upon in “and I stood upon my feet”. This secondary reading is not impossible, which perhaps explains why the incorrect upon was copied into the 1849 LDS edition and the first printing of the 1852 LDS edition. The second printing of the latter edition restored the original unto.
Summary: Maintain the preposition unto in Alma 36:23 (“and did manifest unto the people”).