server <- function(input, output) {

output$contents <- renderTable({

req(input$file1)

df <- read.csv(input$file1$datapath,
               header = input$header,
               sep = input$sep)
df[1,3]=99
df
})
output$downloadData<-downloadHandler(
filename=function(){
  paste("data",".csv",sep="")
},
content=function(file){
  write.csv(df,file)
}
)
}