append

You could also do a custom record field based on the relationship between the Record used in query 1 and the Record used in query 2.

docs.appian.com/.../fnc_crf_customfielddefaultvalue.html

As Stewart said, we are supposed to use a!update () function but if its just one value updating you can use as suggested below but if its many and you want all of them to happen in one single logic then here you go: -

a!localVariables(
local!query1Result: {
a!map(id: 54, class: "B", owner: "xyz"),
a!map(id: 55, class: "C", owner: "abc"),
a!map(id: 56, class: "D", owner: "mno"),
a!map(id: 57, class: "E", owner: "ghi")
},
local!query2Result: { a!map(id: 54, owner: "help"), a!map(id: 57) },
a!forEach(
items: local!query1Result,
expression: a!localVariables(
local!toUpdateIndex: wherecontains(
tointeger(index(fv!item, "id", null)),
tointeger(index(local!query2Result, "id", null))
),
local!toDo: if(
a!isNullOrEmpty(local!toUpdateIndex),
{},
index(
local!query2Result,
local!toUpdateIndex,
null
)
),
if(
a!isNullOrEmpty(local!toDo),
fv!item,
a!update(
fv!item,
"owner",
a!defaultValue(
tostring(index(local!toDo, "owner", null)),
tostring(index(fv!item, "owner", null))
)
)
)
)
)
)

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a!localVariables(
  local!query1Result: {
    a!map(id: 54, class: "B", owner: "xyz"),
    a!map(id: 55, class: "C", owner: "abc"),
    a!map(id: 56, class: "D", owner: "mno"),
    a!map(id: 57, class: "E", owner: "ghi")
  },
  local!query2Result: {
    a!map(id: 54, owner: "help"),
    a!map(id: 57, class: "F")
  },
  local!getIndex: cast(
    typeof({ 1 }),
    index(local!query2Result, "id", null)
  ),
  a!forEach(
    items: local!query1Result,
    expression: if(
      contains(
        local!getIndex,
        tointeger(index(fv!item, "id", null)),

      ),
      a!localVariables(
        local!data:index(
          local!query2Result,
          wherecontains(
            tointeger(index(fv!item, "id", null)),
            local!getIndex
          )
        ),
        local!keys: a!keys(
         cast(typeof(a!map()), 
         index(
            local!query2Result,
            wherecontains(
              tointeger(index(fv!item, "id", null)),
              local!getIndex
            )
          )
         )
        ),
        a!update(
         data: local!data,
        index:  local!keys,
         value:touniformstring(index(
           fv!item,
           {local!keys}
         ))
        )
      ),
      fv!item
    )
  )
)