Now that most distros have moved to grub2, it is important to learn the recommended way to set the default boot option.
Grub2 dynamically generates the configuration file /boot/grub2/grub.cfg. This is done by calling the grub2-mkconfig command which call the scripts in /etc/grub.d/* in sequence and prints the resulting grub2 configuration file onto STDOUT.
The default kernel to boot is set based on the value for directive "GRUB_DEFAULT" in /etc/default/grub. You can set the default to a number as in the grub1 configuration. However it is recommended you set this to 'saved' so that you can use the grub2-set-default command. You use the grub2-set-default command to set the label of the kernel you would like to boot. This is a better approach when compared to using the index number of the kernel listed in the grub configuration file since this avoids problems caused by change in order of the listed kernels.
To set the default kernel,
1) Check to see that the directive "GRUB_DEFAULT" in /etc/default/grub is set to 'saved'.
2) List the kernels present in grub.cfg with the command: grep ^menuentry /boot/grub2/grub.cfg|cut -d "'" -f2
3) Select the kernel label from the list obtained from the previous kernel and then set it as default using the command: grub2-set-default
4) Verify that the correct kernel has been selected with the command: grub2-editenv list
You should then reboot the machine to load the new kernel.