Step 1: After you sign into www.lifecake.com on your computer, click on the 'three dots' button in the top right (circled in red below):
Step 2: Click on 'Family & Friends' (circled in red below)
Step 3: Click on the contact you'd like to change permissions for.
Step 4: You'll be given the ability to change the options to different levels. To understand what each level can do, see this.
Step 5: Click anywhere else on the timeline to save the change.