Install

gem install GEM

Install in ~/.gem.

gem install GEM --user-install

Update

gem update --system

Using --system seems like a common flow.

--system [VERSION]           Update the RubyGems system software

I found I needed root access for /var/lib/gems/2.7.0 directory.

sudo gem update --system

Bundler

$ gem install bundler --user-install

$ gem update bundler --user-install