You need to purchase a third party service to convert data with class to QB Online. I would recommend accessing your historical data in QB Desktop and start using the class feature to register new transactions in your QBO account.

Hello there, @Dhami.

I can see a duplicate post for this exact query you posted. My colleague, RCV, has already answered similar question you've posted. You can view her answer through this thread: Moving data from Desktop to QBO - is the new migration process working yet?

Don't hesitate to keep in touch if you need anything else about exporting from QuickBooks Desktop. As always, the Community is always here to attend to your concerns.