var arr = [
  {
    name: 'hoho',
    items: [
      {
        a : 1
      }
    ]
  }
]

var q = arr.reduce(function(acc, o){
  _.each(o.items, function(item){
    item.a = 2;
  })
  acc.push(o);
  return acc;
});

console.log(q);