Prevent Page refresh on Button Click in ASP.Net

1. Place ScriptManager on Page.

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

2. Add an UpdatePanel and place the Button inside it.

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
     <ContentTemplate>             
           <asp:Label ID="Label1" runat="server" Text=""></asp:Label><br />
           <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />       
     </ContentTemplate>
</asp:UpdatePanel>

3. Click Event.

protected void Button1_Click(object sender, EventArgs e)
{
     Label1.Text = "Button Clicked!";
}