how to edit in my dropdownlist box in asp.net

Please see my comment.

First of all, you should understand how ASP.NET works in a Web application. There are no such HTML elements as "drop-down list" or "text box". ASP.NET code works on the server side and generate pure normal HTML content, without all those ASP.NET Wen controls. In particular, the only control elements where a user can enter some text would be <input type="text"> or <textarea> elements, both are represented by the ASP.NET TextBox control. This is all you can use for editing.

Now, it's up to you to design the code which would present such input element to the user. In principle, you can even mimic in-place editing of a list element, but it would not so simple. When a user edits the text and hit Enter or clicks some button or other control meaning committing the edited text to the drop-down list, you should write some JavaScript to update, say, the selected item of a list to be edited.

This is your job to do. If you try to design and implement something like that and have some particular difficulties you can clearly explain, you are welcome to ask your questions; we will gladly try to help you.

—SA