// you've selected a bunch of text elements that already exist on the page
var text = svg.selectAll("text")
  .data(data); //<-- you've now bound data to them

// text here is the elements already on the page, we just update them
text.attr("class", "update");

// if there's more data then previous existing elements
// these are returned by .enter()
text.enter().append("text")

// now you are operating on both existing and new elements
text.text(function(d) { return d; });

var myCoolRect = svg.append('rect')
  .attr('id', 'myCoolRect')
  .attr('x', 100)
  ....;

myCoolRect.attr('x', 250);

svg.select('#myCoolRect')
  .attr('x', 100);