How to reindex data from 5 to six to upgrade ES to 7

I want to do reindex data which is created in version 5 to version 6 so that we can upgrade ES to version 7

so we are following below

curl 'localhost:9200/_cat/indices?v' -- list all
curl -X PUT "localhost:9200/my_index_block/write?pretty" -- to allow clone
curl -X POST "localhost:9200/my_index/_clone/cloned-my_index?pretty" --- clone old index

then I tried below
curl -X POST "localhost:9200/_reindex?pretty" -H 'Content-Type: application/json' -d'

{
  "source": {
    "index": "my_index"
  },
  "dest": {
    "index": "cloned-my_index"
  }
}
'
but it gave below error 
{
  "took" : 159,
  "timed_out" : false,
  "total" : 9495,
  "updated" : 0,
  "created" : 0,
  "deleted" : 0,
  "batches" : 1,
  "version_conflicts" : 0,
  "noops" : 0,
  "retries" : {
    "bulk" : 0,
    "search" : 0
  },
  "throttled_millis" : 0,
  "requests_per_second" : -1.0,
  "throttled_until_millis" : 0,
  "failures" : [
    {
      "index" : "cloned-my_index",
      "type" : "_doc",
      "id" : "a3f73570-73a8-11ec-a9f3-0a58ac1134f5",
      "cause" : {
        "type" : "cluster_block_exception",
        "reason" : "index [cloned-my_index] blocked by: [FORBIDDEN/8/index write (api)];"
      },
      "status" : 403
    },

can someone help me with steps how I can reindex data from 5 to 6 ,

also I am not sure data in 5 has multiple indexes whereas data in 6 has only single index of that,

also can what does it means
mapping types in your index and elastic 7.0 doesn't supports doc type so you have to reconfigure your data accordingly ,such that type can be removed

Please help me here I am new to ES

Regards,
SAM

Did you mean /my_index/_block/write?pretty? If you do, that sets the index to read only - Index blocks | Elasticsearch Guide [8.1] | Elastic, which I am not sure you want.