I want to split the existing index to daily index

Elastic Stack Elasticsearch
6

You can use _reindex with a painless script to change dynamically the index name based on the date of the event, something like this should work

POST _reindex?wait_for_completion=false
{
  "source": {
    "index": "source-index-name"
  },
  "dest": {
    "index": "destination-index-name"
  },
  "script": {
    "source": """
    
        def inputFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
        def myDate = inputFormat.parse(ctx._source['@timestamp']);
        
        def outputFormat = new SimpleDateFormat("yyyy-MM-dd");
        def outputDay = outputFormat.format(myDate);
        
        ctx._index = "destination-index-name-" + outputDay;
"""
  }
}