After I installed ruby with homebrew in OXS,

brew install ruby
ruby --version

it showed the default (older) version of ruby is still in used.
How to switch the ruby version to the newest one?

i. Close the terminal and restart

There are some experiences shows that after installing ruby with homebrew,
we should try close the terminal(Cmd+Q) and restart it.
Then it may show the new installed version of ruby.

ii. Try using RVM(Ruby Version Management)

After searching the solution of this issue, most of the answer recommand the rvm,
[which you can try too!][https://rvm.io/rvm/install]

However, if you want to continue the path of homwbrew,
you may try the following steps:

1. Check out which version of ruby have you installed?

which ruby
which -a ruby

The first command shows the path of current ruby, and the second one shows the
path of all installed ruby. For instance, you would get:

$ which ruby
/usr/bin/ruby
$ which -a ruby
/usr/bin/ruby
/usr/local/bin/ruby

Homebrews would always take precedence /usr/local/bin.

homebrews should always take precedence

export PATH=/usr/local/bin:$PATH