You would have to go to the Edit Profile page on your LinkedIn app to add or remove skills from the Skills & Endorsements section.
Adding skills in the Skills & Endorsements section:
Open the LinkedIn app in your web browser and find the Me icon at top of the LinkedIn homepage.
Here, click on View profile.
In your profile find the Featured Skills & Endorsement section by scrolling down.
Then to add a new skill click on Add a new skill.
In the Skill text box type the name of the skill you want to add and choose it from the dropdown list. After selecting it, you will find that it gets added to your list of skills automatically.
In case your skill does not appear then you better type the skill name in the field and click Add.
In case you do not find any skills listed on your profile then you will not find the Skills & Endorsements section. For you to add that section:
Find the same Me icon to take you to your profile.
Then click on Add new profile section found on the right side and then select Skills.
Now, you would have to add skills by following the skills.