OpenSearch modify or create new index?

Hello, I have index01 that is created by streaming data. The incoming date field is in milliseconds since the epoch but not being interpreted as a date-time field by OpenSearch. I would like to re-create the entire index (or maybe I can re-create a portion of the index and "Join" with the original index?) and in the process format the date-time to epoch_millis.

I found the below snippet of code which I think I need to use but I have no idea how I would use below to create index02 from index01?

PUT my-index-02
{
  "mappings": {
    "properties": {
      "date": {
        "type":   "date",
        "format": "epoch_millis"
      }
    }
  }
}