how to update specific node in D3 - d3.js

Question

I'm living updating nodes and links. I also want to change the radius of existing nodes(circles). how can i update eg a specific circle?
When i do this below it updates all circles.
node.select("circle").attr("r", circleradius);
when i update the node array nothing changes in the viz.
nodes[index].size = 2000;
this is how the outputted html looks like
this is the function update code:
function update() {
// Restart the force layout.
force
.nodes(nodes)
.links(links)
.start();
// Update links.
link = link.data(links, function(d) { return d.target.id; });
link.exit().remove();
link.enter().insert("line", ".node")
.attr("class", "link");
// Update nodes.
node = node.data(nodes, function(d) { return d.id; });
node.exit().remove();
var nodeEnter = node.enter().append("g")
.attr("class", "node")
.on("click", click)
.call(force.drag);
nodeEnter.append("circle")
.attr("r", function(d) { return Math.sqrt(d.size) / 2 || 24.5; });
nodeEnter.append("text")
.attr("dy", ".35em")
.text(function(d) { return d.name; });
node.select("circle")
.style("fill", color);
}

Answer 1

Child elements are not automatically updated until you use d3.select. It's a bit of a hard thing to wrap your head around which I won't get into here.
But for practical purposes, what you need to do is, after you've created your force-directed layout and elements and such, if you want to change a data value used for size element 0 and see it update, the code needs to read like this:
nodes[0].size = 2000;
d3.selectAll("g").select("circle")
.attr("r", function(d) { return Math.sqrt(d.size) / 2 || 24.5; });