Download the VM2 package, com_virtuemart.2.0.x_extract_first (http://dev.virtuemart.net/projects/virtuemart/files). Unpack the zip on your local PC and then install com_virtuemart.2.0.x.zip using the Joomla Extension Manager. Install the sampla data, too, when prompted.
Then install the modules and plugins package, com_virtuemart_ext_aio.2.0.x.zip - this All In One package installs the VM Joomla modules/plugins and the supplied shipping and payment plugins.
That's it. Note that you must be using a good reliable host.
If you see an error message like "There was a problem uploading the file to the server" it is possible that the component filesize is too large for the php resource settings on the server. If this is so it is often possible to install using targz.zip file packages (if available), as these are smaller.
I recommend to use the same procedure (using Joomla's Extensions Installer) when updating VM versions as it is generally reliable, but as with all updates, test on a backup copy of the site first.
Also see this workthrough, written for an earlier development version of VM2, but still mainly valid: http://www.ostraining.com/blog/joomla/virtuemart/
In subsequent VM versions the release number (eg 2.0.12) may change but the installation procedure is the same.
When updating VirtueMart be sure to install the new component followed by the new aio component.
Note for VM2.6 and later a minimum PHP 5.3.10 is required on the server.