How can I convert my online quickbook files to my desktop version?

The export to desktop (US Only) provides you with a file that must be opened in desktop within 10 days, and the file has known problems, Some things do not export, inventory exports as service items with no history, payroll if you have it is exported as regular checks, these and other known problems have existed for over 9 years
see the limitations:
https://community.intuit.com/articles/1145409-export-limitations

Print out the QBO P&L, balance sheet, inventory valuation summary (if you have inventory), and sales tax liability as a minimum, compare the QBO reports to the same reports in desktop after you have a desktop company file - they must match, if not you have to find and correct the problems, intuit will not help

IMO if at all possible, it would be better to just re-enter all transactions in desktop

how to go from QBO to QBDT
https://community.intuit.com/articles/1145829-exporting-from-online-to-desktop

Hello, desha.

Thanks for taking the time to visit the Community. I'd be happy to show you how to convert from QuickBooks Online to Desktop.

There are a few requirements to note before getting started:

• You must be the Master Administrator, Company Admin, or invited accountant to export to Desktop.
• You need to use a computer equipped with Internet Explorer 10 (or higher).
• The supported Desktop version must be installed on the computer already.
• ActiveX is needed to complete the export from QBO.

I've got an article for you that offers the steps to complete the conversion process. You can check it out here: https://quickbooks.intuit.com/community/Reports-and-accounting/Export-data-file-from-QuickBooks-Onli....

Additionally, this write-up explains the limitations of switching from one QuickBooks product to the other: https://quickbooks.intuit.com/community/Reports-and-accounting/Export-limitations-QuickBooks-Online-....

If you need assistance converting to Desktop, you may wish to speak with a member of the Support Team. Agents have a screen-sharing tool to work along side you every step of the way. The number to speak with an agent is 1-800-488-7330. The most up-to-date contact information can be found here: https://help.quickbooks.intuit.com/en_US/contact.

You're more than welcome to contact me with any questions you may have, as well. I wish you and your business continued success. Take care.