Not able to execute painless scripts

I am quite new to painless or any kind of scripting in elastic.

I wanted to know how do i execute a script in elastic.

Scenario: I have a mapping.json, with all the fields to be indexed in elastic. I am using elasticsearchjs npm module as my code is in typescript. So to execute the script which is basically adding a new field to an existing document. Looks somewhat like this "ctx._source.name = SZ_MK.pdf".

Now my question is where do i add -->

      "inline": "ctx._source.name = SZ_MK.pdf",
      "lang": "painless"
    }

so that it gets executed is it within mappings or somewhere else?

It depends on what you are trying to do. If you want to update a single document, you can use the update api. If you are trying to update all of your documents, then look at update_by_query. And if you want to modify documents as they are indexed then look at ingest scripts.