Renaming legend items in ArcGIS for Desktop?

several ways...
In the layer properties dialog on the general tab you can change the name and description.
in the table of contents select the name and click again. you will be able to type anything you want. You should also note that when in the legend if you slect the layer and thenthe style button changing the style to layer name with label, heading with label, heading with label description. so on so forth...

You can change the values under the Label heading in the Symbology tab (not the Label tab). You can rename the layers in the Table of Contents. You can convert your legend to graphics and do various things with them.

You could also convert the legend to graphics and explode it to individual graphic and text elements. You will loose the dynamic link to the layers but you will be able to easily control all properties of each item in the legend.