Converting a .QBO file into a CSV file is easy peasy. And I'd be happy to guide you through the steps.
Open the .qbo file on your computer.
On the Save as type, choose CSV.
Click Save.
However, if you don't have this option, you can consider looking for a third-party application that can help convert the file.
After converting the file, you may want to import your transactions in QuickBooks Self-Employed manually. For more guidance, feel free to check out this article: Manually import transactions.
Keep in touch if you need any more assistance converting a file, or there's something else I can do for you. I've got your back.