Update part of a document existing field

https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-update.html#_update_part_of_a_document

according to this documents, I can update partial documents. but is it not.

it change whole documents.

and this document explains updating add new documents. but I want to edit existing field.

is it possible?

As Lucene and therefore Elasticsearch stores data in immutable segments there is no in place update. Any change to a document will make it re reindexed behind the scenes no matter how small the change is.