The concept of mass decreasing as we go up is not accurate according to our current understanding of physics. In everyday life, we often associate weight with mass, but they are actually different properties.
Mass refers to the amount of matter an object contains and is a fundamental property of an object that remains constant regardless of its location. According to the theory of general relativity, mass is responsible for the gravitational force an object exerts and experiences. Therefore, an object's mass remains the same regardless of its position in a gravitational field.
Weight, on the other hand, is the force exerted on an object due to gravity. Weight depends on an object's mass and the strength of the gravitational field it is in. As we move higher above the Earth's surface, the gravitational field strength decreases, resulting in a decrease in weight. This is why you may feel lighter on top of a mountain compared to being at sea level.
However, it's important to note that the mass of an object does not change with respect to its position in a gravitational field. The mass remains constant, while the weight changes due to the varying gravitational force.