library(shiny)
library("xlsx")

ui<-fluidPage(
  fileInput("file1","Choose xlsx file",
            accept = c(".xlsx")),
  downloadButton("file2","Download Modified File")
)

server <- function(input, output){

  output$contents <- renderTable({

    req(input$file1)
    f1 <- cbind(read.xlsx(input$file1$datapath,3),
                read.xlsx(input$file1$datapath,4))
  })

  output$downloadData <- downloadHandler(
    filename = function() {
      paste(output$file2$datapath,"xlsx",sep = ".")
    },
    content = function(file) {
       write.xlsx(f1, file)
    }
  )
}

shinyApp(ui=ui, server=server)