To edit an existing list begin by going to the Audience tab in the main navigation bar then click 'Lists.'
Next, choose the list you'd like to edit from the menu.
In the next screen you'll see all the profiles within your list and an 'X' icon to the right of each profile.
Click the x to delete profiles and if you've like to add new ones click 'Add Profile.'
Add Profile to List
You can also add a profile to a list by searching for that profile in the main Search tab, clicking into that profile and selecting the list you'd like to add that profile to from the dropdown menu. All lists highlighted in blue are lists that profile is associated with.