Check & Remove old Ruby & Rails Version?

You can find your ruby and rails versions by running:

ruby -v
rails -v

gem uninstall rails # to uninstall

And to uninstall ruby, you'd just follow the uninstall instructions from rbenv, rvm or whatever you used to install it.

As a side note, you don't want to remove any Ruby that comes pre-installed on your system (like macOS has a Ruby pre-installed) because the operating system depends upon it.