How do I update Ruby from the default version to 2.1.6 via terminal?
Since homebrew has moved on to ruby version 2.4.2 you might not have luck installing that far back a version. 

I've not used RVM lately, since I'm happy with keeping up with the ruby version that's stable (and let home brew update things since I have no legacy projects needing 2.1), but give the following steps a shot. This will side load another version that's older than what Apple ships.


https://rvm.io


Once that's installed:

 rvm install 2.1.6  
 rvm --default use 2.1.6