<telerik:RadToolBar ID="RadToolBar1" runat="server">
    <Items>
        <telerik:RadToolBarButton runat="server" Text="Button 0" Value="button1">
        </telerik:RadToolBarButton>
        <telerik:RadToolBarButton runat="server" Text="Button 1">
        </telerik:RadToolBarButton>
        <telerik:RadToolBarButton runat="server" Text="Button 2">
        </telerik:RadToolBarButton>
        <telerik:RadToolBarButton runat="server" Text="Button 3">
        </telerik:RadToolBarButton>

        <telerik:RadToolBarDropDown runat="server" Text="DropDown 0" Value="MyDropDown">
            <Buttons>
                <telerik:RadToolBarButton runat="server" Text="Child Button 1" Value="value1">
                </telerik:RadToolBarButton>
                <telerik:RadToolBarButton runat="server" Text="Child Button 2" Value="value2">
                </telerik:RadToolBarButton>
                <telerik:RadToolBarButton runat="server" Text="Child Button 3" Value="value3">
                </telerik:RadToolBarButton>
            </Buttons>
        </telerik:RadToolBarDropDown>
    </Items>
</telerik:RadToolBar>

    protected void Page_Load(object sender, EventArgs e)
    {
        var dropdown1 = RadToolBar1.FindItemByValue("MyDropDown") as RadToolBarDropDown;
        dropdown1.Buttons[0].Text = "Changed button";

        var dropdown2 = RadToolBar1.FindDropDownByValue("MyDropDown");
        dropdown2.Buttons.FindItemByValue("value2").Text = "Changed button text from FindDropDownByValue";

        var dropdown3 = RadToolBar1.Items.FindItemByValue("MyDropDown") as RadToolBarDropDown;
        dropdown3.Buttons.FindItemByValue("value3").Text = "Changed button text from Items.FindItemByValue ";

    }

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim dropdown1 = TryCast(RadToolBar1.FindItemByValue("MyDropDown"), RadToolBarDropDown)
    dropdown1.Buttons(0).Text = "Changed button"

    Dim dropdown2 = RadToolBar1.FindDropDownByValue("MyDropDown")
    dropdown2.Buttons.FindItemByValue("value2").Text = "Changed button text from FindDropDownByValue"

    Dim dropdown3 = TryCast(RadToolBar1.Items.FindItemByValue("MyDropDown"), RadToolBarDropDown)
    dropdown3.Buttons.FindItemByValue("value3").Text = "Changed button text from Items.FindItemByValue "

End Sub