Mint 17.1 (Rebecca), is still based on 14.04 LTS release (14.04.1 LTS, Trusty Tahr) so update process only concern Mint own packages.
sudo cp /etc/apt/sources.list.d/official-package-repositories.list /etc/apt/sources.list.d/official-package-repositories.list.bak
sudo sed -i 's/qiana/rebecca/' /etc/apt/sources.list.d/official-package-repositories.list
Be sure to use Mint main US site (http://packages.linuxmint.com). Mirrors sites may not be up to date and you may ruin your update
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get upgrade
### Restart system
Check 17.1 has been installed, issue :
cat /etc/issue
You should see :
Linux Mint 17.1 Rebecca \n \l
Second look in lsb-release
cat /etc/lsb-release
You should see :
DISTRIB_ID=LinuxMint
DISTRIB_RELEASE=17.1
DISTRIB_CODENAME=rebecca
DISTRIB_DESCRIPTION="Linux Mint 17.1 Rebecca"
Kernel (uname -a)
Linux xxxxxxx 3.13.0-37-generic #64-Ubuntu SMP Mon Sep 22 21:28:38 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
You're right
cat /etc/lsb-release
Well, Mint 17.1 is 14.04 LTS based so others sources are not rebecca (17.1) but Ubuntu roots based, and I think they don't need to be disabled