how to update ruby

open Terminal and then run

 \curl -sSL https://get.rvm.io | bash -s stable

(Yelp, there is  a \ at beginning)

restart Terminal to make rvm ( Ruby Version Manager)  to work  

Now, run rvm list known 
This shows the list of versions of the ruby.

use the latest ruby shown for example 2.2.1

run rvm install ruby-2.2.1

If you type ruby -v in the terminal, if it still shows you ruby 2.0 , then run rvm use ruby-2.2.1