I need help regarding update the elastic search document.
From what I know, I don't think you can update existing documents in ES. You may have to reindex the data into new indexes, and if necessary, you can create aliases for new indexes. Certain mappings can be updated, but pretty limited.
I think it's a delete and a re-insert.
Nope, as per @unknownunknown's link you can do it.
Here is an example of using Java API
Even though ES supports "update", it should be used with care because in theory, it's an expensive process. The index has no problem in servicing search requests while updating existing documents but the ranking of the search results will be affected due to the changes of the term-document matrix under the hood.
Don't be surprised to see the expected documents that used to be at the top of the search results now are not there. They may or may not depending on what was updated.