Linkedin will give you suggestions based on your profile informations; you can also type in different keywords to see what will come up.
You can only add pre-filled skills, but there are a very wide range of skills to choose from!
Related to the following Options:
A) If you just want to Add a skill go to your Skill Section and click on Add a new skill.
B) Or if you want to edit and or remove a skill click on the pen to do so.
C) If you want your top skills to show up in the featured skills section of your linkedin profile, make sure that your to 3 skills are pinned under "Top Skills" appear there.