Answer: Hello there!
We want to find a counterexample to: "a square has four right angles" then we need to find a figure that has four right angles and is not a square.
A first example can be a cross (two segments that cut each other and are perpendicular), but let's think in a geometric shape.
Think on a square that in the middle of each side has a small semicircle; this shape still has four right angles but is not more a square.
There you have two counterexamples of the definition given.