Ubuntu

Clean up old kernels

# first make sure you really run the latest kernel. Ideally simply reboot, you normally get the newest then.

# now become root
sudo bash

#search for installed kernels
dpkg --list | grep linux-image 

# check, which kernel you are using (just to be safe!)
uname -r

# remove all older kernels
apt-get purge linux-image-w.x.y.z-generic 

# next clean up headers and other stuff
apt-get autoremove --purge

# finally update grub2 (might not be necessary, as the kernel packages should do it themself
update-grub2 

Resolve Apache 2 error

If you get

AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message

do this

sudo bash
echo "ServerName localhost" > /etc/apache2/conf-available/fqdn.conf
a2enconf fqdn.conf
service apache2 reload