Hello rachel7,
For .QBW file, you can restore this backup copy in your QuickBooks Desktop 2018. Please follow the steps in this article: Restore a backup of your company file.
For .QBX file, you'll get prompted to convert it to the latest version if you'll restoring it from your QB Desktop 2018. From there, you can start working or converting it to QBO.
Please click on this link to learn more how to convert accountant's copy to a working file.
You're always welcome to post some more.