For iOS users to unfriend someone, tap the "Friends" icon at the bottom of the Dashboard.
For Android users, tap the icon with three horizontal lines in the top left of the screen, then select "Friends & Follows" from the menu.
From your list of friends, select the profile you would like to unfriend.
If on iOS, tap the Friends icon with a checkmark by it. A menu will appear with the option to "unfriend."
If on Android, tap on the icon with three vertical dots in the upper right corner of the page. An option to "unfriend" should appear.
After un-friending someone, their posts will no longer appear on your Activity Feed and vice versa.