Switching from QB Online to QB Desktop can be a lengthy process but I'll be happy to help.
Since there's no direct way to move your client's account from QuickBooks Online to QuickBooks Desktop, you'll need to export your data out of QB Online.
The info you can export out of QB Online is limited at this time. If there's important info which you're finding hard to export out of QBO, don't hesitate to reach out for help. Once you've exported you QBO file as CSV files, you can import them to QB Desktop.
To import a list in the CSV format, you need to install the QuickBooks Import Excel and CSV toolkit. The toolkit contains:
Steps to install the QuickBooks Import Excel and CSV toolkit
In case you have other questions, feel free to contact our support team using this link here.