.net - Modify An Asp:BulletedList In C#?

I've different buttons like this

<asp:Button ID="button1" runat="server" Text="single" title="single" EnableViewState="false" />

When this button is clicked I want that the text in a list like this changes

<asp:BulletedList ID="list1" runat="server" EnableViewState="false">
<asp:ListItem>Item1</asp:ListItem>
<asp:ListItem>Item2</asp:ListItem>
<asp:ListItem>Item3</asp:ListItem>
<asp:ListItem>Item4</asp:ListItem>
</asp:BulletedList>

How in C# can I select the first, the second or the third element of the list?

protected button_Click(object sender, EventArgs e)
{
list1. ...;
}