The following steps i took to update the kernel of the 15.04 Ubuntu installation where tvheadend is running.
check the current version with:
uname -r
Update the kernel:
cd /tmp
wget kernel.ubuntu.com/~kernel-ppa/mainline/v4.4.8-wily/linux-headers-4.4.8-040408_4.4.8-040408.201604200335_all.deb
wget kernel.ubuntu.com/~kernel-ppa/mainline/v4.4.8-wily/linux-headers-4.4.8-040408-generic_4.4.8-040408.201604200335_amd64.deb
wget kernel.ubuntu.com/~kernel-ppa/mainline/v4.4.8-wily/linux-image-4.4.8-040408-generic_4.4.8-040408.201604200335_amd64.deb
sudo dpkg -i linux-headers-4.4*.deb linux-image-4.4*.deb
sudo reboot
uname -r to see the new running version of the kernel.