One can't just "update the cache" of web browser. Instead of that the server should include in the HTTP response headers which manage the cache. If you want that the every Ajax request will be get from the server you should include in the server response. For example I include practically always the following header
Cache-Control: private, max-age=0
in the response of the server which provide data for the grid. The answer shows how such headers can be set in ASP.NET.
Here you can read more about HTTP headers used for caching of data. The answer together with the previously referenced provide some details which are interesting in combination with jqGrid and jQuery.ajax.