Update VirtualBox without losing Guest .vdi Files
Shut down all VMs and quit VirtualBox. Open VirtualBox-5.1.6-110634-OSX.dmg and double-click the installer in the mounted image. The installer will overwrite the old VirtualBox.app and some other files.

The VMs will be kept in place and won't be modified.

Later download the latest Extension Pack 5.1.6-110634 and install it in the existing VMs if necessary/possible.