Upgrade Linux Mint 17.3 to 18

Seems that the normal do-release-upgrade thing has been replaced by a smarter script. Try

sudo apt-get install mintupgrade 
mintupgrade check
mintupgrade download
mintupgrade upgrade

From their site, published June 30, 2016:

It will also be possible to upgrade from Linux Mint 17.3. Upgrade instructions will be published next month.

Link: Linux Mint 18 “Sarah” MATE released!

Assuming they do it as they did before, it'll show up as an option in the Mint Update Manager.

I did that:

sudo sed -i 's/rosa/sarah/g' /etc/apt/sources.list.d/official-package-repositories.list
sudo sed -i 's/trusty/xenial/g' /etc/apt/sources.list.d/official-package-repositories.list

And i commented

#deb http://extra.linuxmint.com rosa main

Then

apt update 
apt upgrade
apt dist-upgrade
reboot
apt update && apt upgrade && apt dist-upgrade
sudo apt-get autoremove

→ Everything was fine, but strangely my mintsources remain in "rosa" and not "sarah"/