Can any one one tell me how to change the legend of the graph of crystal report in .net

You'll need to be a little more specific. What do you need to change?

In the Crystal designer, you can right-click on the legend of the chart, and Crystal will display short-cut menus.

If you're data-driving the graph, the text for the legend is generally bound to the column specific in the "On change of" part of the graph.

Kevin