Unlock iPhone 3GS running iOS 6.1 and Downgrade Baseband 6.15.00

OK, the 3GS is a bit dated, but I recently inherited this device from my brother, who I [unsuccessfully] tried to convert to Android with a shiny new Nexus 4. He had been complaining that his 3GS was very slow and wanted the ability to tether to his laptop. He’s on my business plan with …