Hello miss mel,
If you follow the easy upgrade for QuickBooks steps when upgrading to 2019, you don't have to download, install, and register your QuickBooks anymore. It seamlessly updates your company file without any hassle.
Otherwise, after you've downloaded QuickBooks on your computer, you'll have to go through the installation process. You can check out this link on how to install QuickBooks. Lastly, open or restore your company file.
Let us know if you have any questions. We're here to help.