You can't until the final version will be released. 

Hello.

You can go to: https://www.odoo.com/apps and download apps manually. I'm not sure if all of them are going to work since Odoo is making changes these days but you can give a try. You have to put the folder of the module on addons folder. Or you can create your own folder and add that folder to your "addons_path" on openerp_server.conf.

To install these modules you've to give "Technical Settings" permission to your user. And then you have to refresh your browser. You're going to see a link under Configuration named "Update Module List". You've to update your module list from there and you're going to have that modules on your system.

You can Import Zip file from Import Module option.