The velocity at any time is: v = vo + at
Taking up to be the positive direction, a = -g and v = vo + (-gt).
g is the magnitude of the acceleration due to gravity.
As time elapses vo is constant and -gt grows linearly with time, so the velocity v decreases at a constant rate.
Another way to look at this is to use v2 = v1 +
When the acceleration is constant Δv = a Δt= -gΔt.
So, v2 = v1 + (-gΔt).
By stepping the motion forward in equal time steps you can see that the change in velocity is the same for each step.