the only way to upgrade is:
You have to remove/uninstall the old version first
sudo apt remove virtualbox virtualbox-5.2
then
sudo apt update
then install the new file you downloaded by clicking on it,
in my case it was:
virtualbox-6.0_6.0.4-128413~Ubuntu~xenial_amd64.deb
You have to remove/uninstall the old version first
sudo apt remove virtualbox virtualbox-5.2
then
sudo apt update
then install the new file you downloaded by clicking on it,
in my case it was:
virtualbox-6.0_6.0.4-128413~Ubuntu~xenial_amd64.deb