Hello Adrian,
Thanks for contacting us on Community.
Depending on the age of your QuickBooks Online file you can migrate directly from QuickBooks Desktop, to do so select the Company heading in QB Desktop and then select Export Company File to QuickBooks Online. If your file for QuickBooks Online is older than 90 days you may not be able to migrate straight away if this is the case please contact our dedicated support team on the number below and select QuickBooks Desktop.
0808 168 9535
Please let us know if you need any further help.