The developer, Vladimir Romanyuk, has explicitly stated: "The free 0.990 version is provided 'as is' for legacy purposes. Do not repackage it."
If you intended to ask for a between the free 0.980 and the paid 0.990 versions, or instructions for a legitimate installation of SpaceEngine, please clarify, and I will gladly provide that instead.
Space Engine is a 1:1 scale science-based Universe simulator. It features billions of galaxies, nebulae, stars, and planets. Version 0.990 was a massive milestone for the software. It marked the transition from a niche hobbyist project to a professional-grade simulator, introducing:
Moral of the story: Unofficial downloads of old software might not give you a game — they might give you a mystery. Or a warning. Or a doorway you can’t close.