Thanks for following the thread, mckannd. Let me step in and provide a detailed step-by-step guide on how we can upgrade your QuickBooks Desktop (QBDT) from 2015 to the recent version.

First, let's save the QBDT 2015 trial version in the local C drive. The C drive is the system's primary hard drive and is used to store the operating system, system files, and other applications and their associated files. To do that:

1. Locate the file you've downloaded.
2. Right-click on the file and select Copy.
3. Next, let's go to your My Computer to select the Local C Drive. 
4. Click Users, then Public.
5. Right-click on the screen and click Paste.

Once done, let's install the trial version of the QBDT 2023 and restore the file. Please refer to this article for the link to the trial version: Download a trial of QuickBooks Desktop.

Then, here's how to restore the company file:

1. Go to the File menu and click Open or Restore Company.
2. Select Restore a backup copy, then click Next.
3. Click Local Backup, and click Next.
4. Browse your computer to locate the backup company file. It should have an extension of .qbb. 
5. Choose a folder to save your restored company file, then click Open.
6. When ready, please select Save. If you're prompted with any message about overwriting your data, choose the option which best fits your needs. 

Moreover, you mentioned being unable to migrate from QBDT to QuickBooks Online. Don't worry, I've got you an article with a list of common migration errors and suggested steps to fix them: Fix errors when you move from QuickBooks Desktop to QuickBooks Online.

To welcome you to QBO, I'll leave this reference as your guide in navigating the product: Learn about converting from QuickBooks Desktop to QuickBooks Online. It's a compilation of the most frequently asked questions about converting your account from QBDT to QBO. This way, it'll be easier for you to adjust when doing business tasks.

Please don't hesitate to ask for assistance if you have additional concerns about upgrading your QBDT. The Community is always ready to lend you a hand. Stay safe, and have a great day!