indx is the selected index no of datalist which i saved on a lable
var clienttype = document.getElementById('<%=lblDivlastIndx_Itm.ClientID %>').value;
var indx = parseInt(clienttype);
indx=indx+1;
$("#ctl00_ContentPlaceHolder1_dlRouts_ctl0"+indx+"_txtSeatNo").val('New Value');
Now its working fine.