Open $ vim /etc/default/grub then add elevator=noop next to GRUB_CMDLINE_LINUX_DEFAULT. Run $ update-grub and $ cat /sys/block/sda/queue/scheduler to be sure that noop is being used:
$ vim /etc/default/grub
$ update-grub
$ cat /sys/block/sda/queue/scheduler
[noop] deadline cfq