Update database record based on Bootstrap multi select DropDownList selection using C# in ASP.Net

I have a multi select dropdown containg TagID like this

now I want to update the PenID against the multi select TagID.

for example I multiselect the two TagID 102 and 103 and from the dropdown list I select the PenID SKY and perform the update operation.

Simpley, It can be said that I want to perform multiselect update on a single click rather to update one by one row. How it can be done?

my code is 

<div class="row">
    <div class="col-sm-12">
        <label class="control-label" style="margin-top: 10px;">
            Tag ID</label>          
        <asp:DropDownList ID="ddlTag" runat="server" class="form-control">
        </asp:DropDownList>
        <br/>
        <asp:ListBox ID="lstTag" runat="server" SelectionMode="Multiple" class="form-control" >
        </asp:ListBox>              
    </div>
</div>
<div class="row">
    <div class="col-sm-12">
        <label class="control-label" style="margin-top: 10px;">
            Move To</label>
        <asp:DropDownList ID="ddlPen" runat="server" class="form-control">
        </asp:DropDownList>
    </div>
</div>                                  
<div class="row">
    <div class="col-sm-12">
        <label class="control-label" style="margin-top: 10px;">
            Date</label>
        <asp:TextBox ID="txtDate" runat="server" class="form-control" placeholder="Move to Pen Date"></asp:TextBox>
        <asp:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtDate" PopupButtonID="txtDate" Format="dd.MM.yyyy" >
        </asp:CalendarExtender>
    </div>
</div>

protected void btnAdd_Click(object sender, EventArgs e)
{
    // this.BindGridUpdate();
    try
    {
        con = new SqlDbConnect();
        con.SqlQuery("Update tblAnimal set PenID=@PId where TagID=@TId");
        con.Cmd.Parameters.Add(new SqlParameter("@PId", this.ddlPen.SelectedValue.ToString()));
        con.Cmd.Parameters.Add(new SqlParameter("@TId", this.ddlTag.SelectedValue.ToString()));
        con.NonQueryEx();
        con.conClose();
        ClearControls(Page);
        // this.BindGrid();
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Animal Moved to the Pen Successfully.');", true);
        btnAdd.Visible = true;
        btnUpdate.Visible = false;
        btnDelete.Visible = false;
    }
    catch (Exception ex)
    {
        Response.Write(ex.Message);
    }
}