grub settingsfor booting Mint 17.1
 

I want to change the boot order of Mint 17.1 and Fedora 21.

At the moment it boots up to Fedora 21 and I want to make it boot
up Mint 17.1

I understand I have to modify grub, where is it and how do I change it
to boot Mint?

If you are using the Fedora Grub bootloader to boot both Fedora and Mint, you would boot Fedora and go to the /etc/defaul/grub file and look for the line below. It should be right at the top of the file. You would need to change this entry. To determine what to change it to, you would need to look at the grub.cfg file and count the menuentry lines down to the one which lists Mint and enter that number if this file. Count in this instance is from zero, not from one.

That can be fragile - personally I'd just boot to Mint and run grub-install.

boot of mint
 

Sorry, I don't know what that means does it make mint the first to boot?

Yes, each distro puts itself first - it will also have entries for the Fedora kernels.
grub-install re-writes the MBR so Mint "owns" it. This is what happens if for example you had installed F21 first, then installed Mint.

grub on mint
 

I ran grub-install on a terminal and it said "operation not permitted" and "device not specified"
What should the command be?

Assuming you only have one hard drive it would be

I've used GRUB Customizer without a hitch:

https://launchpad.net/~danielrichter...rub-customizer