(1) Check the versions available.
rbenv install -l
(2) If you can't find the latest version, you need to update rbenv.
brew upgrade rbenv ruby-build
(3) Now install the version you want. (This will take some time.)
rbenv install 2.4.4
(4) Set it as global.
rbenv global 2.4.4
(5) Install bundler gem and other gems as necessary.
gem install bundler rbenv rehash
(6) To uninstall the older version, use this command.
rbenv uninstall 2.4.2