First you must be on the 'edit page' for the profile. You can get there from the profile... on the top right of the screen hover over the name of the page to get the drop-down-box from which you select 'edit'. On that edit page you will see all the blanks to put information... the one you want is not a box like the others. Just above the one holding their name you will see the little green link to change the LNAB. I believe this is what you were looking for... along with the info provided by Ann B.
Good luck to you,
I should add...I understand that changing the LNAB is to be avoided if at all possible. Certainly be sure about it.