composer

Drush 9.x Install Composer to your LOCAL user directory

cd ~
mkdir .composer
curl -sS https://getcomposer.org/installer | php
mv composer.phar ~/bin/composer
chmod +x ~/bin/composer
echo "PATH=$PATH:~/.composer/vendor/bin" >> ~/.bash_profile
. ~/.bash_profile

Install latest version of Drush locally

Get the latest (or whatever) version here:

https://github.com/drush-ops/drush/releases

cd /LOCATION/OF/DRUPAL
composer require drush/drush

And we done...