It is highly recommended reading these articles (and more help) if you are converting from or to QBO. Note: This is not a seamless process. Expect too many hurdles to overcome!
Import limitations from QuickBooks desktop (QBDT) version to QuickBooks Online (QBO).
What doesn’t come over during an import from QuickBooks Desktop – Import Limitations (US only). Click here.
Export limitations from QuickBooks Online (QBO) to QuickBooks desktop version (QBDT).
Export limitations for QuickBooks Online to QuickBooks Desktop. Click here to see this article.
Export limitations… more
For Export from QBDT to QBO:
Import into QuickBooks Online from QuickBooks Desktop for Windows (US only). Click here to see this article.
The Accountant’s Guide to Moving Clients QuickBooks Online (QBO). HERE.
For Export from QBO to QBDT:
Visual Guide to Export..here
Exporting from QBO to QuickBooks desktop editions.. here
Convert QuickBooks Online company file to QuickBooks desktop.. here