How to update to latest VirtualBox and backup files?
Solution 1:
Similar question here
https://askubuntu.com/questions/19268/how-to-upgrade-virtualbox-to-the-latest-version
Technically you will not lose your virtual machines by just upgrading vbox
Solution 2:
Don't worry, you won't loose your VMs (I just updated yesterday). If you really want, you could copy your VDI file (the one containing your hard drive) to another location as backup, and in case something goes wrong, you could make a new Ubuntu VM, and use your backed up VDI as your hard drive.