// Define the cancellation token source & token as global objects 
CancellationTokenSource source = new         CancellationTokenSource();
CancellationToken token = null;

//when button is clicked, call method to run task & include the cancellation token 

private async void button1_Click(object sender, EventArgs e)
{
  token = source.Token;
  await Backup(file, token);
}


public async Task Backup
(string File, CancellationToken token)
{
  Task t1 = Task.Run(()  =>
  {
    //do something here
  }, token);
} 

//cancel button click event handler 
private async void cancelButton_Click(object sender, EventArgs e)
{
  if(source! = null) 
  {
    source.Cancel() 
  } 
}

//tasks
https://msdn.microsoft.com/en-us/library/system.threading.tasks.task(v=vs.110).aspx 

//CancellationToken
https://msdn.microsoft.com/en-us/library/system.threading.cancellationtoken(v=vs.110).aspx