FATAL Error: Index .kibana belongs to a version of Kibana that cannot be automatically migrated. Reset it or use the X-Pack upgrade assistant

[Hooman_Bahreini]

I have installed elasticsearch cluster on a Linux server. Initially I installed Elasticsearch 7.x (but then changed the elasticsearch to 6.8). Elasticsearch is running fine.

I did the same with Kibana on a Windows Server. initially installed version 7.x then uninstalled it and installed Kibana 6.8

When I run Kibana I get this error:

.....
istant.
    at assertIsSupportedIndex (C:\kibana-6.8.0-windows-x86_64\src\server\saved_objects\migrations\core\elastic_index.js:246:15)
    at Object.fetchInfo (C:\kibana-6.8.0-windows-x86_64\src\server\saved_objects\migrations\core\elastic_index.js:52:12)
  isBoom: true,
  isServer: true,
  data: null,
  output:
   { statusCode: 500,
     payload:
      { statusCode: 500,
        error: 'Internal Server Error',
        message: 'An internal server error occurred' },
     headers: {} },
  reformat: [Function],
  [Symbol(SavedObjectsClientErrorCode)]: 'SavedObjectsClient/generalError' }

 FATAL  Error: Index .kibana belongs to a version of Kibana that cannot be automatically migrated. Reset it or use the X-Pack upgrade assistant.

ERROR: The process with PID 6020 (child process of PID 6100) could not be terminated.
Reason: There is no running instance of the task.

I have no idea how to reset .kibana index or use x-pack upgrade assistance.

[Hooman_Bahreini]

I managed to fix it by delete .kibana index

curl -XDELETE http://localhost:9200/.kibana -u elastic:password