ReportObjects RepObjCollection = rdViewer.ReportDefinition.ReportObjects;
CrystalDecisions.CrystalReports.Engine.TextObject Text;
CrystalDecisions.CrystalReports.Engine.FieldHeadingObject Header;

foreach (CrystalDecisions.CrystalReports.Engine.ReportObject Obj in RepObjCollection)
{
    switch(Obj.Kind.ToString())
    {
        case "TextObject":
            Text = (CrystalDecisions.CrystalReports.Engine.TextObject)Obj;
            Text.Text = "Your string";
            break;

        case "FieldHeadingObject":
            Header = (CrystalDecisions.CrystalReports.Engine.FieldHeadingObject)Obj;
            Header.Text = "Your string";
            break;
    }
}