You just need to add some event listeners in your JavaScript section to handle the math when those buttons are clicked. Something like this should get you close:

$(document).ready(function(){
  $('#q103 .cf-collection').on('click', '.cf-collection-delete', function(){
    $('#q301 input').val((parseNumber($('#q301 input').val())-1).toString());
  });
  $('#q103 .cf-collection-append').on('click',function(){
    $('#q301 input').val((parseNumber($('#q301 input').val())+1).toString());
  });
});

function parseNumber(n) {
  var f = parseFloat(n); //Convert to float number.
  return isNaN(f) ? 0 : f; //treat invalid input as 0;
}