Try this
Suppose following is your file upload control and textbox in which you want to show file name uploaded in file upload control.
<asp:FileUpload ID="FileUpload1" runat="server" onchange="showFileName(this)" />
<br />
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
Following is java script code.
<script>
function showFileName(input) {
var file = $("#FileUpload1").val();
document.getElementById("TextBox1").value = file;
}
</script>
Simply write one function in java script and call this function on onchange of fileupload control.