How to upgrade/downgrade Rubygems and Bundler:

Upgrade Recipe

$ gem update --system

$ gem install bundler
$ bundle update --bundler

Downgrade Recipe

$ gem update --system 3.7.2

$ gem install bundler -v 2.7.2
$ bundle update --bundler=2.7.2

Install a pre-release version

$ gem update --system --pre

$ gem install bundler --pre
$ bundle update --bundler=4.0.0.beta1

Install from source

• Download from: https://rubygems.org/pages/download
• Unpack into a directory and cd there
• Install with: ruby setup.rb