Once skydivers jump out of the plane, they start to accelerate due to the gravitational force exerting on them. They will continue to accelerate until they reach the point where the gravitational force is equal to the drag force (or air resistance) acting on them. At that point, they are moving at "terminal speed". At this point, the forces acting on them balance out so the net force is equal to zero.

Picture retrieved from http://www.physicsclassroom.com/mmedia/newtlaws/sd.cfm

Fnet= ma
0 = ma
Therefore, a = 0 m/s^2
​(no accerelation)

Picture retrieved from http://www.real-world-physics-problems.com/physics-of-skydiving.html

m = mass 
W = weight = mg
vt  = terminal speed 
v  = speed at which he falls 
D = drag force (air resistance)
g = acceleration due to gravity (9.8 m/s^2)

It's important to note that skydivers can change their speed by changing the cross-sectional area. This can be done in many ways. For example, in the spread-eagle position shown in the figure below, the skydivers typically reach the speed of 120 mph.

Picture retrieved from https://www.flickr.com/photos/divemasterking2000/3186255286/

Now, if they want to change their speed so that they could fall faster, they would have to increase their speed by reducing the cross-sectional area. One way to do that is to put their heads straight down. 

Eventually, the skydivers would want to release their parachutes so that they can land safely. Parachutes have a very large cross-sectional area, which is why they allow us to slow down and land at a speed where we wouldn't get injured. 

​

Picture retrieved from https://www.flickr.com/photos/divemasterking2000/3397473576/