Renaming fields in elasticsearch

The Request field does not yet exist in your documents, so your script needs to create it first:

POST _reindex
{
    "source": {
        "index": "testindex"
    },
    "dest": {
        "index": "testindex1"
    },
    "script": {
        "inline": "ctx._source.Request = [:]; ctx._source.Request.field3 = ctx._source.remove(\"field2\") ]"
    }
}

Or a bit shorter like this:

POST _reindex
{
    "source": {
        "index": "testindex"
    },
    "dest": {
        "index": "testindex1"
    },
    "script": {
        "inline": "ctx._source.Request = [field3: ctx._source.remove(\"field2\") ]"
    }
}