How to upgrade ember-cli globally to run ember serve of the latest

I'm trying to get Ember-cli globally to the latest version so that when I run ember new it actually installs the latest version. I've followed the instructions here: https://cli.emberjs.com/release/basic-use/upgrading/

npm uninstall -g ember-cli
npm install -g ember-cli
ember --version

And the version (v2.15) does not reflect the change (v3.7). I've also checked the version just after uninstalling and it still shows up as if it hadn't been removed. I've also tried running npm cache clean after the uninstall.

I'm running ubuntu 16.04

I've also tried following the similar question here: How to upgrade ember cli version and it does NOT solve my problem.

Is there some way to select what version to use when running ember new? Or change the global version selected?