I want to download a copy of quickbooks desktop to install a client's backup to confirm the migrated data brought into QBO as their data is a mess, can it be done?

Ask your client. Which QB Desktop year version they have.

Let's make sure you'll be able to download the trial version, office257.

Switching from QuickBooks Desktop to QuickBooks Online (QBO) transfers much information from your company file without having to re-enter it manually. When you start to download QuickBooks Desktop, make sure your computer meets the minimum requirements. Once, done, you can download it through this link.

However, some information may not convert or convert differently. To know more about these, please visit this article: What To Expect When You Switch From QuickBooks Desktop to Online. Also, to ensure that the migration process is correct, I've provided this article for your guide: Migrating From QuickBooks Desktop to QuickBooks Online. Move Your QuickBooks Desktop File to QuickBooks Online.

For your reference, here's an article that you can check out so you'll be guided when using QBO: QBO Help Articles You Can Check.

Let us know if you have any concerns with the migration. We're here to help you.