replace specific name in layer in adobe illustrator

The above script loops through all the layers and names each layer, as it is written to do. 

Layers don't have a "selected" property, but they do have a "hasSelectedArtwork" property.  One can add a conditional to test whether or not a layer "hasSelectedArtwork", limiting the naming to those layers which do.

var document = app.activeDocument;
var layers = document.layers;
for (var i = 0; i < layers.length; i++) {
    if (layers[i].hasSelectedArtwork == true) {
        layers[i].name = "copyfx";
    }
}