Trying to install Jekyll on my Macbook I noticed that the version shipped with Mojave is 2.3.something. There are a couple of ways to solve this.

homebrew

The first one is using homebrew. Provided that you have installed it before, it is just a command away:

brew install ruby

Don’t forget to set the PATH environment to make sure that you use the right Ruby version:

export PATH=/usr/local/opt/ruby/bin:$PATH
ruby -v
ruby 2.6.4p104 (2019-08-28 revision 67798) [x86_64-darwin18]

rbenv

rbenv is a tool commonly used to manage different versions of Ruby, which is quite useful sometimes. Of course it can be installed from homebrew as well:

brew install rbenv
rbenv init

# Add rbenv to you bash shell environment
echo 'eval "$(rbenv init -)"' >> ~/.bash_profile

After that, you can install whatever version of Ruby you need. For example:

rbenv install 2.6.3
rbenv global 2.6.3
ruby -v
ruby 2.6.3p62 (2019-04-16 revision 67580)