You have two options
From Their Profile
Click the Following button on their profile card to unfollow them.
From Connections
Go to Connections in the menu.
Select the Following category.
Find the person in the list, click the three dots next to their name, and select Unfollow.
After unfollowing, you won’t see their content or updates in your feed, but you’ll remain friends, meaning you can still message each other and see their birthday. You can follow them again at any time.