Hello, Mike. WikiTree is set up so only the profile manager or someone on the trusted list can change the last name at birth. You should contact the profile manager and explain your evidence supporting the last name you've found; then the PM can change the LNAB from Unknown to the actual surname. Alternatively, you can request to be added to the "trusted list" for the profiled person, and if approved by the PM, you can make the changes (making sure to add your evidence supporting the new LNAB.)