I have been using QBO as a trial but I now want to go back to QBDT. How do I properly transfer my file back to QBDT?

Thanks for asking the Community today, @Nothingventured. I've got you the first out of six steps in exporting QuickBooks Online (QBO) data to QuickBooks Desktop (QBDT).

Here's how:

1. Log in to the QuickBooks Online account.
2. Highlight the URL and replace anything after /app/ with exportqbodatatoqbdt.  The URL should say https://cxx.qbo.intuit.com/app/exportqbodatatoqbdt. XX in the URL is a number, (don't edit the number), then select Enter on your keyboard. Other support task links won't work during this process.
3. Read the Export Overview once you'll see an Export Overview Window, then select Get Started.

Once done, proceed with Step 2: Download QuickBooks Desktop and the migration tool outlined in this article: Export your QuickBooks Online data to QuickBooks Desktop.

You also want to learn about some limitations when exporting data from QBO to QBDT by reading through this article: Export limitations - QuickBooks Online to QuickBooks Desktop.

I'm always around here to help if you have more questions. Have a great day ahead. 

@Nothingventured 

For your additional reference

https://quickbooks.intuit.com/learn-support/en-us/account-management/trying-to-export-online-back-to...