Please read the Learning Centre: Experimenting with ISO

You may need to read your camera user manual regarding ISO settings and how to change them.

Go out and take two photos, by setting your camera to Aperture mode (as per the first challenge), and selecting a suitable aperture of your choice. Once selected, do not change your aperture.

Take your first photo at ISO 100
Take your second photo at ISO 1600 (or the closest to 1600 your camera will allow)

Present both photos here and tell us what the differences are. Also consider what your camera did differently to get a correct exposure, between the two photos.