If you already have VirtualBox installed on Debian, it only takes a couple of minutes to upgrade to the latest version.
In this case, I will be upgrading Debian Stretch from v6.0.x to v6.1.x
All the prerequisites on https://www.virtualbox.org/wiki/Linux_Downloads have been met since this is an upgrade.
Open Terminal and run these commands:
sudo apt-get update sudo apt remove virtualbox virtualbox-6.0 sudo apt-get install virtualbox-6.1
Once the installation is finished, run the VirtualBox Manager. It will prompt you to install the matching v6.1x Extension Pack.
As you open each Guest, run the Guest Additions to upgrade it as well.