This code shows how to swap two rows in a datagrid.
var index1 = 2;
var index2 = 3;
var dg = $('#dg');
var rows = dg.datagrid('getRows');
var temp = rows[index1];
rows[index1] = rows[index2];
rows[index2] = temp;
dg.datagrid('refreshRow',index1);
dg.datagrid('refreshRow',index2);