It happened to me when i first learn about Rails . I read a lot of Rails books with different versions of Rails each book. It is not efficient if we Install and Uninstall Ruby and Rails for several times because of the version of Rails we read from the books. Thank GOD we have RVM in this beautiful world .

If you want to know how to install it , it's not the right place for that , just Google it :)

See the commands below to see the way we can use different Ruby versions and so does the Rails :