I'm a little rusty on tutorials, but here is a tutorial on upscaling pixel art. Note that these methods I go through do not preserve the alpha channel.
Game Jam: https://itch.io/jam/game-creation-org...
Kopf–Lischinski algorithm article: http://www.extremetech.com/gaming/840...
xBRZ: https://sourceforge.net/projects/xbrz/
Super xBR: http://www.libretro.com/