Go to https://www.odoo.com/page/download

& Fill the form then download and Install , Setup includes all stuff Python,PostGRES means everything included in package. Simple Download and Run the exe file and follow the simple steps.

You can donwload directly the latest installer for Windows at odoo_8.0-latest.exe any time & install/reinstall it. This link always points to latest version at nightly repository of openerp, practically it's updated almost every day.

UPDATE:
in Debian/Ubuntu you can install/update using apt-get as it explained at: http://nightly.odoo.com/

in Ubuntu you've to run commands:

- for installation

1) sudo echo 'deb http://nightly.odoo.com/8.0/nightly/deb/ ./ '  > /etc/apt/sources.list.d/odoo8.list

2) sudo apt-get update

3) sudo apt-get install odoo

- for update:

1) sudo apt-get update

2) sudo apt-get upgrade

- for start/stop/restart/status:

 sudo service odoo <here start or stop or restart or status >