How do you define the row that you want to update? I’m going to assume that is the row that you have selected, and the name of the column being updated is symbol.
// Get a reference to the grid
var grid = $("#my_grid").data("kendoGrid");
// Access the row that is selected
var select = grid.select();
// and now the data
var data = grid.dataItem(select);
// update the column `symbol` and set its value to `HPQ`
data.set("symbol", "HPQ");
Remember that the content of the DataSource is an observable object, meaning that you can update it using set and the change should be reflected magically in the grid.