ON THE EDIT PAGE OF THE PROFILE sitting between the "middle name" slot and the Current Last Name slot is the AREA (NOT a slot) that reads "Last Name at Birth" <surname> and a ? mark in a green circle and [edit] in green print ... this is on the left side of the screen
You will encounter WARNINGS bout changing the LNAB when you click on [edit] so be prepared and you will need a concise explanation like "new information discovered"