// Get a service used to work with express report
var eaxMdService = expressBox.getService();
// Get express report document
var eaxDocument = expressBox.getSource();
// Get key of express report active sheet
var sheetKey = eaxDocument.getActiveSheetKey();
var sheetName = "New name"; // Determine a new sheet name
var onSheetRenamed = function (n, t) {
    expressBox.getDataView().getSource().getSheets()[t.Args.SheetKey].setName(t.Args.SheetName),
    expressBox.getDataView().refresh(),
    expressBox.getDataView().DocDataChanged.fire(this, new PP.Mb.Ui.PropertyChangedEventArgs({
        PropertyName: PP.Exp.Ui.ControlType.DataView,
        Metadata: null,
        TypeUpdateData: [PP.Exp.Ui.ViewTypeUpdate.Ribbon, PP.Exp.Ui.ViewTypeUpdate.PropertyBar, PP.Exp.Ui.ViewTypeUpdate.Title]        
    }))
};
eaxMdService.renameSheet(eaxDocument, sheetKey, sheetName, PP.Delegate(onSheetRenamed, this, {
    SheetKey: sheetKey,
    SheetName: sheetName
}));
// Refresh express report
expressBox.refreshAll();