Answer
Navigate to the Welcome page itself first (using the blue dot at the very start of the breadcrumb trail) so it appears in Recent Screens — sometimes it takes navigating back and forth between the Welcome page and another page once or twice before it actually shows up there. Once the Welcome page appears in Recent Screens, set it as the homepage from there using the Home icon, exactly the same way any other page would be set as homepage; there's no separate "clear homepage" action, but setting the Welcome page itself as homepage achieves the same practical result.