Even if you went to space and saw a sphere earth that does not mean that the earth is a sphere. it just means that the light warped so much when it left the atmosphere that your brain thought it was round.
At least three things will happen, though.
1. You would see the continental layout of the planet as it passes under you, and even if you believe it's distorted by an enormous amount of light warping you'll be able to map the planet more accurately than a flat earth has been mapped before.
2. You would watch the earth rotate beneath you and will be able to learn much about how the sun illuminates it.
3. You could see the sun set behind the earth without any magnification effect of an atmosphere and get a better idea of the size of the sun.
It may not definitively answer a round or flat earth question, but you would certainly be able to learn a lot more about the earth, moon, sun and cosmos from the ISS.