Hello Ján,
If you would like to re-fetch the data and display a new aggregated result, then you will need to reset the source. This will force the widget to fetch the new data, perform the calculation and to display the updated result. You can find more details how to reset the source in this how-to demo:
http://docs.telerik.com/kendo-ui/controls/data-management/pivotgrid/how-to/change-data-source
I hope this will help to achieve the desired result.
Regards,
Stefan
Telerik by Progress