Merge two buckets muli_level inside buckets

[Azizi_BESSEM]

{
  "aggregations" : {
    "alert_types" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "1",
          "doc_count" : 3,
          "device_refs" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : "over_voltage",
                "doc_count" : 2
              },
              {
                "key" : "under_voltage",
                "doc_count" : 1
              }
            ]
          }
        },
        {
          "key" : "2",
          "doc_count" : 3,
          "device_refs" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : "over_voltage",
                "doc_count" : 2
              },
              {
                "key" : "off_ligne",
                "doc_count" : 1
              }
            ]
          }
        }
      ]
    }
  }
}

i want to merge the two bucket in one bucket because i need to paginate the result , please your support team