Grub2 is the bootloader in RHEL/CENTOS distributions. Grub2 boot parameters are stored in /etc/default/grub.

Edit /etc/default/grub to change timeout from 5 to 2 seconds:

# vi /etc/default/grub
    GRUB_TIMEOUT=2
    [...]

Update grub2 configuration with the new parameters:

# grub2-mkconfig -o /boot/grub2/grub.cfg
# reboot

Set grub2 to default boot on the 2nd entry of the kernel list:

# grub-set-default 1