Refer below code.

C#

protected void OnSubmit(object sender, EventArgs e)
{
    string values = hfSelectedEmails.Value.Replace(',' , '/');
    // Write code to insert in database.
}

VB.Net

Protected Sub OnSubmit(ByVal sender As Object, ByVal e As EventArgs)
    Dim values As String = hfSelectedEmails.Value.Replace(","c, "/"c)
    ' Write code to insert in database.
End Sub