how to update the record of employee

<div class="form-horizontal">
               <div class="form-group">
                   <label class="col-sm-2 control-label">
                       Roll No <span class="symbol required"></span>
                   </label>
                   <div class="col-sm-3">
                      <asp:DropDownList CssClass="form-control search-select" runat="server" MaxLength="7" ID="ddlRollNo"  />
                      <asp:RequiredFieldValidator ValidationGroup="vgEmployee" runat="server" ID="rfvrOLL" ControlToValidate="ddlRollNo" Required="required"></asp:RequiredFieldValidator>
                    </div>
                   <label class="col-sm-2 control-label">
                       Rank <span class="symbol required"></span>
                   </label>
                   <div class="col-sm-3">
                       <asp:DropDownList CssClass="form-control" ID="ddlRank" runat="server"  />
                       <asp:RegularExpressionValidator runat="server" ID="revddlRank" ControlToValidate="ddlRank" Required="required" Display="Dynamic"></asp:RegularExpressionValidator>
                   </div>
               </div>
               <div class="form-group">
                   <label class="col-sm-2 control-label">
                       Start Date:
                   </label>
                   <div class="col-sm-3">
                       <asp:TextBox ID="txtStartDate" runat="server" MaxLength="10" Required="required" data-date-viewmode="years"
                           CssClass="form-control date-picker" data-date-format="dd-mm-yyyy"></asp:TextBox>
                    </div>
                   <label class="col-sm-2 control-label">
                       Start Time:
                   </label>
                   <div class="input-group input-append bootstrap-timepicker col-sm-3">
                       <input id="txtstartTime" runat="server" type="text" class="form-control time-picker" />
                       <span class="input-group-addon add-on"></span>
                   </div>
                   </div>
               <div class="form-group">
                   <label class="col-sm-2 control-label">
                       End Date:
                   </label>
                   <div class="col-sm-3">
                       <asp:TextBox ID="txtEndDate" runat="server"  Display="Dynamic"
                           CssClass="form-control date-picker" OnTextChanged="txtStartDate_TextChanged" AutoPostBack="true" data-date-viewmode="years" Required="required" MaxLength="10" data-date-format="dd-mm-yyyy"></asp:TextBox>
                      </div>
                   <label class="col-sm-2 control-label">
                       End Time:
                   </label>
                   <div class="input-group input-append bootstrap-timepicker col-sm-3">
                       <input id="txtEndTime" runat="server" type="text" class="form-control time-picker"  />
                       <span class="input-group-addon add-on">^__i class="fa fa-clock-o"></span>
                   </div>
               </div>
               <div class="form-group">
                   <label class="col-sm-2 control-label">
                       Durtion:
                   </label>
                   <div class="col-sm-3">
                       <asp:TextBox CssClass="form-control"  runat="server" ID="txtDurtion" Required="required" />
                       <asp:RegularExpressionValidator runat="server" ID="revDurtion" ControlToValidate="txtDurtion" ></asp:RegularExpressionValidator><br />
                   </div
                       <label class="col-sm-2 control-label">
                       Select Sector:
                   </label>
                   <div class="col-sm-3">
                       <asp:DropDownList CssClass="form-control" ID="ddlSelectSector" runat="server"  />
                       <asp:RegularExpressionValidator runat="server" ID="revSelectSector" ControlToValidate="ddlSelectSector" Required="required" Display="Dynamic"></asp:RegularExpressionValidator>
                   </div>

try
          {
              string[] temp = txtstartTime.Value.Split(new char[] { ' ', ':' });
              DateTime startDate = SafeConvert.ToDateTime(txtStartDate.Text).AddHours(temp[2] == "PM" ? SafeConvert.ToInt32(temp[0]) + 12 : SafeConvert.ToInt32(temp[0]))
                  .AddMinutes(SafeConvert.ToInt32(temp[1]));

              temp = txtEndTime.Value.Split(new char[] { ' ', ':' });
              DateTime endDate = SafeConvert.ToDateTime(txtEndDate.Text).AddHours(temp[2] == "PM" ? SafeConvert.ToInt32(temp[0]) + 12 : SafeConvert.ToInt32(temp[0]))
                  .AddMinutes(SafeConvert.ToInt32(temp[1]));

              EmployeeAbsent empAbsent = new EmployeeAbsent();
              empAbsent.UserName = MyUser.UserName;
              empAbsent.EntryDate = DateTime.Now;
              empAbsent.Emlopyee_Id = ddlRollNo.Text;
              empAbsent.Rank_Id = SafeConvert.ToByte(ddlRank.SelectedValue);
              empAbsent.StartDate = startDate;
              empAbsent.EndDate = endDate;
              empAbsent.Durtion = txtDurtion.Text;
              empAbsent.Sector_Id = SafeConvert.ToByte(ddlSelectSector.SelectedValue);
              _service.InsertEmployeeAbsent(empAbsent);
              _queryStatus = empAbsent.Id > 0;