GP6 - I cannot, or I don't know how to download the updates or the RSE soundbanks

The updates as well as the RSE banks are available via the Updater integrated into Guitar Pro 6. 
(Menu 'Help > Launch Updater' on Windows; or 'Guitar Pro > Launch Updater' on Mac)

If you wish to launch the Updater without starting Guitar Pro:

On Windows, click on Start (bottom left corner of your screen), then on My Computer.
Click on C:, open 'Program Files' (or 'Program Files(x86)' if you are on a 64-bit machine), and then in the 'Guitar Pro 6' folder right-click on GPUpdater and choose 'Open as administrator'.
This will launch the Updater. In the list of available items, you can choose to download the latest GP6 update, the RSE soundbanks, language packs, etc.

If you are on a Mac computer, use the following link to download a program that will launch the Updater for you: