how to change the ruby and rails version

changing ruby version 

    checking ruby version run(if you had have install rvm to manage rubies:

   $rvm list

   $rvm use ruby-2.0.0-p643 --defualt

changing rails version:

    changing the rails versions when running rails new to build a new app(installing rails in gem):

    $gem list  rails 

    $rails new _3.0.5_ appName