Although all the extant textual sources here read upon (𝓞 is not extant), the critical text will reinterpret this as the more literal up on. Compare this with the original use of up upon two verses later:
We also have the following example of up upon in the next book:
These two examples suggest that we could alternatively emend Alma 62:20 to read up upon rather than up on. Here we accept the simpler emendation, up on. For further discussion of cases where upon should be reinterpreted as either up on or up upon, see under 2 Nephi 4:24–25 and Ether 3:1.
Summary: Emend upon in Alma 62:20 to read more literally as up on.
Alma 62:20, page 2845, line 4
As explained just above, the line here in the citation of Alma 62:20 should read as up on rather than as upon: “and came up on the top of the wall to spy out”.
Alma 62:22, page 2845
In this passage, the probable reason the 1830 typesetter omitted the adverb up from the predicate “and come up upon the top of the wall” is that two verses earlier the text read “and came upon the top of the wall”—that is, without the up. But as explained above in the addendum for Alma 62:20, the preposition upon in verse 20 should actually be interpreted more literally as up on.