First of all, shut off the pertinent VM.
Choose File -> Virtual Media Manager… in Oracle VM VirtualBox Manager. Here you choose the correct disk for your machine and change the size appropriately.
Then boot your Kubuntu VM. Since KDE Partition Manager is NOT able to resize the partition although the OS is aware of the extended storage, you have to
apt-get install gparted
Of course, you could also do this on the command line but using GParted is easy, isn’t it? And it works great! So, that’s all you have to do, resize, apply the changes and reboot.