First, install nodejs

Debian/Ubuntu Linux

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
sudo apt-get update && sudo apt-get install yarn

CentOS / Fedora / RHEL

sudo wget https://dl.yarnpkg.com/rpm/yarn.repo -O /etc/yum.repos.d/yarn.repo
sudo yum install yarn

macOS

$ brew install yarn

upgrade

brew upgrade yarn

uninstall

Check how you installed yarn (by homebrew or npm)

$ brew ls yarn
/usr/local/Cellar/yarn/1.21.1/bin/yarn
/usr/local/Cellar/yarn/1.21.1/bin/yarnpkg
/usr/local/Cellar/yarn/1.21.1/libexec/bin/ (5 files)
/usr/local/Cellar/yarn/1.21.1/libexec/lib/ (2 files)
/usr/local/Cellar/yarn/1.21.1/libexec/package.json

$ npm ls yarn -g
npm info it worked if it ends with ok
npm info using npm@6.13.4
npm info using node@v12.14.0
/usr/local/lib
└── (empty)

npm timing npm Completed in 2067ms
npm info ok

If yarn is installed from Homebrew, then run this

$ brew uninstall yarn
Uninstalling /usr/local/Cellar/yarn/1.21.1... (14 files, 5MB)

See also