How to switch back to the old kernel?
First of all, it must be installed. You can then select it in GRUB's menu
(GRUB is Trisquel's default bootloader). If a newer kernel is installed,
then the newer kernel boots, by default. You can either remove all newer
kernels, or set the GRUB_DEFAULT variable in /etc/default/grub (that you can
edit executing, for instance, 'sudo pluma /etc/default/grub' in a terminal):
https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html
I want to install virtualbox for ubuntu 16.04, because I want to install
32-bit windows 10, because I cannot play a lot of DVDs.
You can play DRM-defective DVDs on Trisquel. To do so, libdvdcss is
required. It is free software, under the GNU GPL.
https://code.videolan.org/videolan/libdvdcss hosts the code. However,
Trisquel does not distribute the related libdvd-pkg package, probably because
it is in Ubuntu's multiverse repository. You can download it from
https://packages.ubuntu.com/xenial/all/libdvd-pkg/download and install it (by
default, GDebi is installed and allows to simply double click on
"libdvd-pkg_1.4.0-1-1_all.deb" in the file manager).
