How to upgrade version of elasticsearch 5.5.1 to the opensearch 2.11.0

While upgrading from the version 5.5.1 to the opensearch 2.11.0 i am getting the follwing error{ "error": { "root_cause": [ { "type": "parsing_exception", "reason": "Unknown key for a VALUE_STRING in [type].", "line": 1, "col": 9 } ], "type": "x_content_parse_exception", "reason": "[1:107] [reindex] failed to parse field [source]", "caused_by": { "type": "parsing_exception", "reason": "Unknown key for a VALUE_STRING in [type].", "line": 1, "col": 9 } }, "status": 400 }