How to update a field type of existing index in Elasticsearch

Thank you very much for your time ! Indeed I found the solution here

https://www.elastic.co/guide/en/logstash/current/plugins-filters-mutate.html#plugins-filters-mutate-convert

and here

https://www.elastic.co/guide/en/logstash/current/plugins-filters-csv.html#plugins-filters-csv-convert