Here the adverb up has been lost at two different times, in π by scribe 2 and later in the 1874 RLDS edition. The RLDS text has maintained the shorter reading. Oliver Cowdery, when he proofed π against π, restored the up. The critical text will maintain the up after the verb stand. For further discussion regarding the loss of up before upon, see under Alma 2:15.
Summary: Maintain in 3 Nephi 20:2 the adverb up in the conjoined predicate βand stand up upon their feetβ, the reading of the earliest textual sources (the 1830 edition and the corrected reading in π).