How to handle a button event in ASP.NET Web Forms and update the UI without reloading the page

You can put the data in Session and after postback retrieve the Session value and set to the respective controls.

Refer below example.

HTML

<table>
    <tr>
        <td>Name</td>
        <td><asp:TextBox ID="txtName" runat="server" /></td>
    </tr>
    <tr>
        <td>Age</td>
        <td><asp:TextBox ID="txtAge" runat="server" /></td>
    </tr>
    <tr>
        <td colspan="2"><asp:Button ID="btnSave" Text="Save" runat="server" OnClick="OnSave" /></td>
    </tr>
</table>

Property Class

C#

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

VB.Net

Public Class Person
    Public Property Name As String
    Public Property Age As Integer
End Class

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        if (Session["Person"] != null)
        {
            Person person = (Person)Session["Person"];
            txtName.Text = person.Name;
            txtAge.Text = person.Age.ToString();
        }
    }
}

protected void OnSave(object sender, EventArgs e)
{
    Person person = new Person();
    person.Name = txtName.Text;
    person.Age = int.Parse(txtAge.Text);
    Session["Person"] = person;

    // Code for operation with the data.

    Response.Redirect(Request.Url.AbsoluteUri);
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        If Session("Person") IsNot Nothing Then
            Dim person As Person = CType(Session("Person"), Person)
            txtName.Text = person.Name
            txtAge.Text = person.Age.ToString()
        End If
    End If
End Sub

Protected Sub OnSave(sender As Object, e As EventArgs)
    Dim person As Person = New Person()
    person.Name = txtName.Text
    person.Age = Integer.Parse(txtAge.Text)
    Session("Person") = person

    'Code for operation with the data.

    Response.Redirect(Request.Url.AbsoluteUri)
End Sub

Screenshot