Answer:
We know that the gravitational force between two objects of mass M1 and M2 that are at a distance R, is given by:
F = G*(M1*M2)/R^2
Where G is a constant.
If you reduce one of the masses, then the gravitational force between the objects will change.
So if we take un account the Earth and the Sun, when you reduce the mass of Earth, the force between Earth and the Sun will decrease, and this will change the orbit of the Earth around the Sun.
(The orbit also depends on the gravitational force between the Earth and the other planets in the system, and all those forces also change, which also has an impact in the orbit change)