In the skill window (K) click on a skill to get a description and preview video. Just below the sill icon, to the right of the circle with the current skill level, there is a small tab with an arrow icon to add skill points (and if you do, there will be an arrow to the left to remove points) and you can confirm your selection(s) with the Accept button at the bottom right of the window, below the skill description. There will not be an arrow for specific skills if you are at the current level cap for those skills, or for any if you use all your skill points.
