how to value change in text box when we change the value in drop down list

you can do the same from aspx page also using javascript.

follow the  below code

<asp:dropdownlist id="ddlreport" runat="server" onclick="fnconchange()" xmlns:asp="#unknown">
<asp:textbox id="txtname" runat="server"></asp:textbox>


<script type="text/javascript">
function fnconchange()
{
var ddlReport = document.getElementById("<%=ddlreport.ClientID%>");
       var Text = ddlReport.options[ddlReport.selectedIndex].text;
       var Value = ddlReport.options[ddlReport.selectedIndex].value;
if(Value =="ram")//(your selected value
{
document.getElementById("<%=textBox.ClientID%>").value = "ram";
}

}
</script></asp:dropdownlist>