Uhhhh...
I can't.
I can't because of the risk of losing my game files. If I launch GM2, a message will appear saying I need to remove the old engine, but if I remove the old GM engine, I might just lose my game files.
The situation turned out to be rather amusing (or perhaps not): I wanted to create the RPG of my dreams. I stumbled upon a game jam dedicated to the games participants wished to bring to life. I created a demo (or rather, a rough draft) called Paintland, but I wanted to make a full-fledged game that offered more than just five minutes of gameplay, so I began developing DREAMWORLD last August. However... my engine broke: it stopped working properly and wouldn't even launch. Consequently, I had to port the concept to a new engine that offered greater development capabilities. Work on the first chapter took a full year. The game may still contain bugs, but I am actively fixing them.