Change All Values in a Dictionary to False

You can try something similar to below:

a!localVariables(
  local!items: {a!map(isActive: true), a!map(isActive: false()), a!map(isActive: true)},
  a!linkField(
    links: a!dynamicLink(
      label: "RESET",
      saveInto: {
        a!save(
          local!items.isActive,
          repeat( length(local!items), false())
        )
      }
    )
  )
)

What version of Appian are you running? There is the a!update() function that allows you to update individual attributes. Here's an example:

a!localVariables(
  local!data: {
    { id: 1, name: "A", isActive: true },
    { id: 2, name: "B", isActive: true }
  },
  a!forEach(
    items: local!data,
    expression: a!update(fv!item, "isActive", false)
  )
)

...and the result is:

[id:1,name:A,isActive:false]; [id:2,name:B,isActive:false]