Press [E] on an upgrade cabinet to use it. Then press [W] and [S] to select which upgrade you want. Press [Space] to buy it, then press [E] again when you are finished buying every upgrade you want. At the top, next to the face with the Xed out eyes, you can see the number of upgrade points you have. You gain one upgrade point every time you get a kill, and lose one when you die (in the next update you won't lose any when you die, but for now that is the case).