There are two ways to uninstall a particular version of Ruby. The easiest is to simply remove the directory from ~/.rbenv/versions:

$ rm -rf ~/.rbenv/versions/2.1.0

Alternatively, you can use the uninstall command, which does exactly the same thing:

$ rbenv uninstall 2.1.0

If this version happens to be in use somewhere, you’ll need to update your global or local version. To revert to the version that’s first in your path (usually the default provided by your system) use:

$ rbenv global system