Version 0.990 is often written as 0990 in download filenames (e.g., SpaceEngine_0_990_40.zip ). The term "upd" refers to the last available update patches (0.990.39.xxx or 0.990.40).
Go to the in-game settings and allocate a larger graphics cache if your GPU has high VRAM. This keeps nearby solar systems pre-loaded for seamless travel. space engine 0990 download free upd