This can be done in 2 ways:
Either go to Manage > Skills> People or go person's dashboard by clicking on their name in the People Planner.
Navigate to the Skills section.
Click on the three-dot menu next to a skill
Select Remove Skill
Select the Skills tab and then open the skill.
Select the person you need the skill removed from and click on the 3 dots.
Click on remove from Skill.