Remove the specified row and then insert it again to where you want. The code below shows how to move a selected row to the top.
var dg = $('#dg');
var row = dg.datagrid('getSelected');
var index = dg.datagrid('getRowIndex', row);
if (index > 0){
dg.datagrid('deleteRow', index);
dg.datagrid('insertRow', {index:0,row:row})
}