Science is a process, not the accumulated knowledge we have thus far.
It's like building a map - the map might change over time and mistakes might even be made that later get retracted and altered, and there might be disagreements about some regions of the map, but that doesn't alter the fact that the territory exists and that, in fact, we have managed to map out much of it and that there is a process by which it makes sense to continue to do so (the scientific method). We are building a map of reality, our universe, Nature, what is true and what is not. There's no a priori reason to believe this territory actually exists, but it definitely seems to, and we've had great success with our map-making so far (see literally all technology).
Saying science is about making up theories at random (something I heard honestly asserted by a fellow trainee teacher this year), or pointing out how scientists now believe different things (have a different map) than scientists in the past as if that invalidates the whole thing, is a misunderstanding of this fact. Science is the answer to the question 'how do we build a conceptual map of reality'? Or, in simpler terms, how do we find out about the world.