Navigate to the top menu bar and click on (Windows) or Guitar Pro (Mac). Select Check for Updates... from the drop-down menu.
While you cannot "download" third-party soundbanks in the same format as RSE, you can expand your sonic palette by: Guitar Pro 7 & 6 – Support
Simply downloading the new soundbanks is the first step. To make your music sound truly professional, consider these tips:
Accurate palm mutes, slides, bends, harmonics, and hammer-ons/pull-offs.