我按照这些指南将 Linux Mint 17.3 升级到 18：
我已经完成了mintupgrade部分之前的所有步骤。这是我尝试过的：
$ sudo apt install mintupgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package mintupgrade
$ sudo apt-get install mintupgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package mintupgrade
我的软件来源指向：
Main (quina) http://packages.linuxmint.com
Base (trusty) http://archive/ubuntu.com/ubuntu
我如何获得 mintupgrade？