update_dropdown_input(
  session,
  input_id,
  choices = NULL,
  choices_value = choices,
  value = NULL
)

if (interactive()) {

library(shiny)
library(shiny.semantic)

ui <- function() {
  shinyUI(
    semanticPage(
      title = "Dropdown example",
      dropdown_input("simple_dropdown", LETTERS[1:5], value = "A", type = "selection multiple"),
      p("Selected letter:"),
      textOutput("selected_letter"),
      shiny.semantic::actionButton("simple_button", "Update input to D")
    )
  )
}

server <- shinyServer(function(input, output, session) {
  output$selected_letter <- renderText(paste(input[["simple_dropdown"]], collapse = ", "))

  observeEvent(input$simple_button, {
    update_dropdown(session, "simple_dropdown", value = "D")
  })
})

shinyApp(ui = ui(), server = server)

}