Prerequisite

Please install Homebrew(official) on your mac.

Install rbenv and ruby-build

At first, I recommend you to install rbenv and ruby-build. They enables you to control Ruby versions flexibly.

brew install rbenv ruby-build

Although I heard that this way tends to yield(don't catch up with the newest version of ruby), it is simple.

Initialize rbenv

Just execute below

rbenv init

This command will add eval "$(rbenv init -)" to your ~/.bash_profile.
Please check it like this

less ~/.bash_profile

If you can't find out the line export PATH="$HOME/.rbenv/bin:$PATH" and eval "$(rbenv init -)", please add it by yourself.

export PATH="$HOME/.rbenv/bin:$PATH"
eval "$(rbenv init -)"

After that, restart your terminal or reload your .bash_profile with this command.

source ~/.bash_profile

And you can check you could install rbenv or not by this command

rbenv -v

This command will return the version of rbenv

Choose the ruby version and install it.

OK, now we can get the list of the versions of Ruby with below.

rbenv install -l

This time, I assume that we chose 2.5.0 as the target.

rbenv install 2.5.0

Many commands will be executed.
After they are finished, try this.

rbenv versions

and you can get like this

* system
  2.5.0

This indicates you have Ruby 2.5.0 but you are using the pre-installed Ruby.
So, let's change to the target ruby with below

rbenv global 2.5.0

Check your rbenv

For now, your mac is ready to use the Ruby 2.5.0, but I recommend you to check the status of your rbenv.

curl -fsSL https://github.com/rbenv/rbenv-installer/raw/master/bin/rbenv-doctor | bash

This will indicate the install status of rbenv.