Anything that's upside down is reversed so that its top part is on the bottom, and vice versa. Before you turn that soda bottle upside down, make sure the top is screwed on tight!
If you want to experience being upside down, you might do a handstand or hang from your knees on a jungle gym. There's also a figurative way to be upside down, when things feel confused: "My world turned upside down when we moved across the country." This phrase has evolved over the years, from up so down in the early 14th century to upsadown around 1400, and finally upside down by the late 1600s.