Run this in your terminal to get the latest Composer version:
curl -sS https://getcomposer.org/installer | phpOr if you don't have curl:
php -r "readfile('https://getcomposer.org/installer');" | phpThis installer script will simply check some php.ini settings, warn you if they are set incorrectly, and then download the latest composer.phar in the current directory

Installer Options

--install-dir

You can install composer to a specific directory by using the --install-dir option and providing a target directory. Options must be appended to -- so that PHP ignores them, like -- --install-dir=bin, example:
curl -sS https://getcomposer.org/installer | php -- --install-dir=bin

--filename

You can specify the filename (default: composer.phar) using the --filename option. example:
curl -sS https://getcomposer.org/installer | php -- --filename=composer

--version

You can install composer to a specific release by using the --version option and providing a target release. example:
curl -sS https://getcomposer.org/installer | php -- --version=1.0.0-alpha8

Windows Installer

The installer will download composer for you and set up your PATH environment variable so you can simply call composer from any directory.
Download and run Composer-Setup.exe - it will install the latest composer version whenever it is executed.

Manual Download

If you prefer to download the phar manually, here are the available versions:
Latest Snapshot
1.0.0-alpha11 (2015-11-14)
1.0.0-alpha10 (2015-04-14)
1.0.0-alpha9 (2014-12-07)
1.0.0-alpha8 (2014-01-06)
1.0.0-alpha7 (2013-05-04)
1.0.0-alpha6 (2012-10-23)
1.0.0-alpha5 (2012-08-18)
1.0.0-alpha4 (2012-07-04)
1.0.0-alpha3 (2012-05-13)
1.0.0-alpha2 (2012-04-03)
1.0.0-alpha1 (2012-03-01)