$ bundle --version
Bundler version 1.17.3

# Pass in the version you wish to use, surrounded by underscores (_)

$ bundle _2.1.4_ --version
Bundler version 2.1.4

Caveats

Multiple major versions of bundler will collide. Keep only one installation of each major version (One bundler 1.*, one bundler 2.*).

To verify the installed versions of bundler

$ gem list bundler

bundler (default: 2.1.4, 1.17.3)