how to manually set column value in kendo grid

Always use set() method from the Model to change values. No need to refresh(), e.g.:

var dataItem = $("#grid").data("kendoGrid").dataSource.data()[0];
dataItem.set("reason", "new value");

Demo

Based on validation is correct or not this is how you can update the first column value.

for (var i = 0; i < data.length; i++) {   
    //access record using for loop - here i is value from loop
    var firstItem = $('#grid').data().kendoGrid.dataSource.data()[i];

    //set col reason value value
    firstItem["reason"]="username length"; 

    //refresh the grid
    $('#grid').data('kendoGrid').refresh();  
}    

Another way is to use dataitem set method as described in this post

The dataItem.set() method is VERY SLOW as it triggers an event each time. Even a list of 100 is notable slow.

To handle larger updates then use

dataItem[field] = value

The downside is that no dirty markers will be applied and the grid will not reflect the changes.

$('#grid').data('kendoGrid').refresh();