How to restart ubuntu using shortcut key

shortcut key to restart pc.

step 1, press n hold alt+print screen, keep holding these 2 till u completed below steps

step 2, R  
step 3, E  
step 4, I  
step 5, S 
step 6, U 
step 7, B
step 8, DONE!

For shutdown
step 1, press n hold alt+print screen, keep holding these 2 till u completed below steps
step 2, R
step 3, E
step 4, I
step 5, S
step 6, U
step 7, O - (To shut down system immediately)
step 8, DONE!

unRaw - (To switch the keyboard from raw mode to XLATE mode)
 tErminate - (To send the SIGTERM signal to all processes except init),
 kIll - (To send the SIGKILL signal to all processes except init)
  Sync - (To sync all mounted filesystems)
  Unmount - (To remount all mounted filesystems in read-only mode to prevent a fsck at reboot)
reBoot. - (To immediately reboot the system, without unmounting partitions or syncing)

read more about this magic key at 

http://en.wikipedia.org/wiki/Magic_SysRq_key

How to remove old kernel

cd /boot/

ls /vmlinuz*

sudo apt-get remove linux-image-[kernel version]-generic

sudo update-grub

*use this command to see what kernel you using.

$ uname -r

How to set boot sector for Grub

$sudo apt-get install startupmanager

Go to System > Adminitration > start up manager > boot option > default operating system