Though acceleration can be any value, the acceleration that occurs due to Earth's gravity is always fixed. The Earth always pulls an object down towards its surface with an acceleration of $$9.8 \, \mathrm{m/s^2}$$.
Remember that going up is positive and coming down is negative. So, acceleration due to Earth's gravity is always $$-9.8 \, \mathrm{m/s^2}$$ When an object is moving upwards, its displacement and velocity will be positive. When an object is moving downwards, its displacement and velocity will be negative. The signs of displacement and velocity change, but, that of acceleration is always $$-9.8 \, \mathrm{m/s^2}$$.