Once you open the download, drag and drop the Backup and Sync icon into the applications folder. Once it's done copying search for Backup and Sync in Finder and open it. If you see a warning that says this application was downloaded from the internet, ignore it and click Open.
WARNING: If you have Drive File Stream installed, a popup will show at top of your screen asking if you'd like to use both or just use File Stream to sync your My Drive. We suggest that you choose to use File Stream for My Drive as this will prevent Backup and Sync from making a separate local folder of your files that will also sync with your Drive. If you choose to use both, we highly suggest that you only use one of those options for updating your My Drive.
If you click on the new cloud icon in the menu bar at the top of your screen, you will see a few of the latest files that were synced to your Drive. If you click on the settings button (3 vertical dots), and click preferences, it will open up a window showing you which folders are currently set to backup to your Drive. Under the Google Drive section you may select to sync your My Drive to your computer but we do not recommend this if you intend to use Drive File Stream.
Now every time something is updated in your backup folders, it will update in your My Drive.
Items will not be removed automatically from Google Drive if they are deleted from the backup folders unless you choose to do so in the preferences menu.