Understanding MBR and EFI
BIOS/MBR is an older partition table type also referred to as Legacy mode. Nowadays, UEFI is in most of the new computer (especially those that came with W8 or W8.1 pre-installed). UEFI always use the gpt partition table type.
To find out each one your computer uses, you can boot into your computers Bios/firmware. Besides, to see if it is UEFI:
$ efibootmgr
In a UEFI install, the grub.cfg file belongs in /boot/efi/EFI/fedora/grub.cfg. In a msdos/MBR install, grub.cfg belongs in /boot/grub2/grub.cfg.
Extra: to see your partitions, you can type:
$ gdisk -l /dev/sda
Modifying Grub Options
Edit /etc/default/grub. For example, setting:
GRUB_DEFAULT="0"
After that, type the grub2-mkconfig command. In a MBR boot:
$ grub2-mkconfig -o /boot/grub2/grub.cfg
In an EFI boot:
$ grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg