How do I upgrade Rails?

I am new to Ruby and Rails, as well as all this command line melarky. So please bear with me.

I have installed, RubyGems, Homebrew, RVM, Rails etc, by following various tutorials.

I think I have all of this going now, but when I type:

rails --version

I get, 3.2.12. I notice that Rails 4 is out. How do I upgrade to this version?

If I do:

gem install rails

Even doing:

gem update rails

I still get 3.2.12.

Any ideas?

UPDATE

When trying a few options here, I get this message returned...

Michaels-MacBook-Pro:~ ParanoidAndroid$ gem install rails --version=4.0
Building native extensions.  This could take a while...
ERROR:  Error installing rails:
    ERROR: Failed to build gem native extension.

    /usr/local/rvm/rubies/ruby-1.9.3-p392/bin/ruby extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/usr/local/rvm/rubies/ruby-1.9.3-p392/bin/ruby
    --with-atomic_reference-dir
    --without-atomic_reference-dir
    --with-atomic_reference-include
    --without-atomic_reference-include=${atomic_reference-dir}/include
    --with-atomic_reference-lib
    --without-atomic_reference-lib=${atomic_reference-dir}/lib
/usr/local/rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
    from /usr/local/rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:461:in `try_link0'
    from /usr/local/rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/mkmf.rb:712:in `try_run'
    from extconf.rb:26:in `<main>'


Gem files will remain installed in /usr/local/rvm/gems/ruby-1.9.3-p392/gems/atomic-1.1.14 for inspection.
Results logged to /usr/local/rvm/gems/ruby-1.9.3-p392/gems/atomic-1.1.14/ext/gem_make.out

Seems there's an error happening. Any further ideas?

Thanks again everyone!

Solution

If you are using RVM then you should create a seperate gemset first like:

rvm gemset create whateverName

in this example I will install rails 4

rvm install 2.0.0

rvm list  

rvm 2.0.0

^^^^(you may need to copy and paste the version number exactly here as it was displayed in rvm list)

rvm gemset create rails4

rvm gemset use rails4

add the following to your gemfile for existing apps

gem 'rails', '4.0.0'

then run

bundle update rails

or do following to just install the gem

gem install rails --version=4.0

UPDATE

If you havne't installed Xcode 4.5 CLI tools then you will need to

Preferences > Downloads > Components

DMG

https://developer.apple.com/downloads

you may then also need to use homebrew to update gcc

brew install apple-gcc42

the following link has all the information you will need:

https://thoughtbot.com/blog/the-hitchhikers-guide-to-riding-a-mountain-lion